util-lib: make timestamp generation and parsing reversible (#3869)

This patch improves parsing and generation of timestamps and calendar
specifications in two ways:

- The week day is now always printed in the abbreviated English form, instead
  of the locale's setting. This makes sure we can always parse the week day
  again, even if the locale is changed. Given that we don't follow locale
  settings for printing timestamps in any other way either (for example, we
  always use 24h syntax in order to make uniform parsing possible), it only
  makes sense to also stick to a generic, non-localized form for the timestamp,
  too.

- When parsing a timestamp, the local timezone (in its DST or non-DST name)
  may be specified, in addition to "UTC". Other timezones are still not
  supported however (not because we wouldn't want to, but mostly because libc
  offers no nice API for that). In itself this brings no new features, however
  it ensures that any locally formatted timestamp's timezone is also parsable
  again.

These two changes ensure that the output of format_timestamp() may always be
passed to parse_timestamp() and results in the original input. The related
flavours for usec/UTC also work accordingly. Calendar specifications are
extended in a similar way.

The man page is updated accordingly, in particular this removes the claim that
timestamps systemd prints wouldn't be parsable by systemd. They are now.

The man page previously showed invalid timestamps as examples. This has been
removed, as the man page shouldn't be a unit test, where such negative examples
would be useful. The man page also no longer mentions the names of internal
functions, such as format_timestamp_us() or UNIX error codes such as EINVAL.

7 files changed, 307 insertions, 104 deletions

diff --git a/man/systemd.time.xml b/man/systemd.time.xml
index aae3accb6c..47229b4a4e 100644
--- a/man/systemd.time.xml
+++ b/man/systemd.time.xml
@@ -57,14 +57,13 @@
   <refsect1>
     <title>Displaying Time Spans</title>
 
-    <para>Time spans refer to time durations. On display, systemd will
-    present time spans as a space-separated series of time values each
-    suffixed by a time unit.</para>
+    <para>Time spans refer to time durations. On display, systemd will present time spans as a space-separated series
+    of time values each suffixed by a time unit. Example:</para>
 
     <programlisting>2h 30min</programlisting>
 
-    <para>All specified time values are meant to be added up. The
-    above hence refers to 150 minutes.</para>
+    <para>All specified time values are meant to be added up. The above hence refers to 150 minutes. Display is
+    locale-independent, only English names for the time units are used.</para>
   </refsect1>
 
   <refsect1>
@@ -83,13 +82,13 @@
       <listitem><para>days, day, d</para></listitem>
       <listitem><para>weeks, week, w</para></listitem>
       <listitem><para>months, month, M (defined as 30.44 days)</para></listitem>
-      <listitem><para>years, year, y (define as 365.25 days)</para></listitem>
+      <listitem><para>years, year, y (defined as 365.25 days)</para></listitem>
     </itemizedlist>
 
-    <para>If no time unit is specified, generally seconds are assumed,
-    but some exceptions exist and are marked as such. In a few cases
-    <literal>ns</literal>, <literal>nsec</literal> is accepted too,
-    where the granularity of the time span allows for this.</para>
+    <para>If no time unit is specified, generally seconds are assumed, but some exceptions exist and are marked as
+    such. In a few cases <literal>ns</literal>, <literal>nsec</literal> is accepted too, where the granularity of the
+    time span permits this. Parsing is generally locale-independent, non-English names for the time units are not
+    accepted.</para>
 
     <para>Examples for valid time span specifications:</para>
 
@@ -110,30 +109,29 @@
 
     <programlisting>Fri 2012-11-23 23:02:15 CET</programlisting>
 
-    <para>The weekday is printed according to the locale choice of the
-    user.</para>
+    <para>The weekday is printed in the abbreviated English language form. The formatting is locale-independent.</para>
+
+    <para>In some cases timestamps are shown in the UTC timezone instead of the local timezone, which is indicated via
+    the <literal>UTC</literal> timezone specifier in the output.</para>
+
+    <para>In some cases timestamps are shown with microsecond granularity. In this case the sub-second remainder is
+    separated by a full stop from the seconds component.</para>
   </refsect1>
 
   <refsect1>
     <title>Parsing Timestamps</title>
 
-    <para>When parsing, systemd will accept a similar syntax, but
-    expects no timezone specification, unless it is given as the
-    literal string "UTC". In this case, the time is considered in UTC,
-    otherwise in the local timezone. The weekday specification is
-    optional, but when the weekday is specified, it must either be in
-    the abbreviated (<literal>Wed</literal>) or non-abbreviated
-    (<literal>Wednesday</literal>) English language form (case does
-    not matter), and is not subject to the locale choice of the user.
-    Either the date, or the time part may be omitted, in which case
-    the current date or 00:00:00, respectively, is assumed. The seconds
-    component of the time may also be omitted, in which case ":00" is
-    assumed. Year numbers may be specified in full or may be
-    abbreviated (omitting the century).</para>
-
-    <para>A timestamp is considered invalid if a weekday is specified
-    and the date does not actually match the specified day of the
-    week.</para>
+    <para>When parsing, systemd will accept a similar syntax, but expects no timezone specification, unless it is given
+    as the literal string <literal>UTC</literal> (for the UTC timezone) or is specified to be the locally configured
+    timezone. Other timezones than the local and UTC are not supported. The weekday specification is optional, but when
+    the weekday is specified, it must either be in the abbreviated (<literal>Wed</literal>) or non-abbreviated
+    (<literal>Wednesday</literal>) English language form (case does not matter), and is not subject to the locale
+    choice of the user.  Either the date, or the time part may be omitted, in which case the current date or 00:00:00,
+    respectively, is assumed. The seconds component of the time may also be omitted, in which case ":00" is
+    assumed. Year numbers may be specified in full or may be abbreviated (omitting the century).</para>
+
+    <para>A timestamp is considered invalid if a weekday is specified and the date does not match the specified day of
+    the week.</para>
 
     <para>When parsing, systemd will also accept a few special
     placeholders instead of timestamps: <literal>now</literal> may be
@@ -167,8 +165,6 @@
              2012-11-23 → Fri 2012-11-23 00:00:00
                12-11-23 → Fri 2012-11-23 00:00:00
                11:12:13 → Fri 2012-11-23 11:12:13
-       11:12:13.9900009 → Fri 2012-11-23 11:12:13
-                          format_timestamp_us: Fri 2012-11-23 11:12:13.990000
                   11:12 → Fri 2012-11-23 11:12:00
                     now → Fri 2012-11-23 18:15:22
                   today → Fri 2012-11-23 00:00:00
@@ -176,28 +172,25 @@
               yesterday → Fri 2012-11-22 00:00:00
                tomorrow → Fri 2012-11-24 00:00:00
                +3h30min → Fri 2012-11-23 21:45:22
-           +3h30min UTC → -EINVAL
                     -5s → Fri 2012-11-23 18:15:17
               11min ago → Fri 2012-11-23 18:04:22
-          11min ago UTC → -EINVAL
             @1395716396 → Tue 2014-03-25 03:59:56</programlisting>
 
-    <para>Note that timestamps printed by systemd will not be parsed
-    correctly by systemd, as the timezone specification is not
-    accepted, and printing timestamps is subject to locale settings
-    for the weekday, while parsing only accepts English weekday
-    names.</para>
+    <para>Note that timestamps displayed by remote systems with a non-matching timezone are usually not parsable
+    locally, as the timezone component is not understood (unless it happens to be <literal>UTC</literal>).</para>
 
-    <para>In some cases, systemd will display a relative timestamp
-    (relative to the current time, or the time of invocation of the
-    command) instead or in addition to an absolute timestamp as
-    described above. A relative timestamp is formatted as
-    follows:</para>
+    <para>Timestamps may also be specified with microsecond granularity. The sub-second remainder is expected separated
+    by a full stop from the seconds component. Example:</para>
+
+    <programlisting>2014-03-25 03:59:56.654563</programlisting>
+
+    <para>In some cases, systemd will display a relative timestamp (relative to the current time, or the time of
+    invocation of the command) instead of or in addition to an absolute timestamp as described above. A relative
+    timestamp is formatted as follows:</para>
 
-    <para>2 months 5 days ago</para>
+    <programlisting>2 months 5 days ago</programlisting>
 
-    <para>Note that any relative timestamp will also parse correctly
-    where a timestamp is expected. (see above)</para>
+    <para>Note that a relative timestamp is also accepted where a timestamp is expected (see above).</para>
   </refsect1>
 
   <refsect1>
@@ -239,8 +232,9 @@
     second component is not specified, <literal>:00</literal> is
     assumed.</para>
 
-    <para>A timezone specification is not expected, unless it is given
-    as the literal string "UTC", similarly to timestamps.</para>
+    <para>A timezone specification is not expected, unless it is given as the literal string <literal>UTC</literal>, or
+    the local timezone, similar to the supported syntax of timestamps (see above). Non-local timezones except for UTC
+    are not supported.</para>
 
     <para>The special expressions
     <literal>minutely</literal>,
@@ -263,38 +257,38 @@
     <para>Examples for valid timestamps and their
     normalized form:</para>
 
-<programlisting>   Sat,Thu,Mon..Wed,Sat..Sun → Mon..Thu,Sat,Sun *-*-* 00:00:00
-     Mon,Sun 12-*-* 2,1:23 → Mon,Sun 2012-*-* 01,02:23:00
-                   Wed *-1 → Wed *-*-01 00:00:00
+<programlisting>  Sat,Thu,Mon..Wed,Sat..Sun → Mon..Thu,Sat,Sun *-*-* 00:00:00
+      Mon,Sun 12-*-* 2,1:23 → Mon,Sun 2012-*-* 01,02:23:00
+                    Wed *-1 → Wed *-*-01 00:00:00
            Wed..Wed,Wed *-1 → Wed *-*-01 00:00:00
-                Wed, 17:48 → Wed *-*-* 17:48:00
+                 Wed, 17:48 → Wed *-*-* 17:48:00
 Wed..Sat,Tue 12-10-15 1:2:3 → Tue..Sat 2012-10-15 01:02:03
-               *-*-7 0:0:0 → *-*-07 00:00:00
-                     10-15 → *-10-15 00:00:00
-       monday *-12-* 17:00 → Mon *-12-* 17:00:00
- Mon,Fri *-*-3,1,2 *:30:45 → Mon,Fri *-*-01,02,03 *:30:45
-      12,14,13,12:20,10,30 → *-*-* 12,13,14:10,20,30:00
-           12..14:10,20,30 → *-*-* 12,13,14:10,20,30:00
- mon,fri *-1/2-1,3 *:30:45 → Mon,Fri *-01/2-01,03 *:30:45
-            03-05 08:05:40 → *-03-05 08:05:40
-                  08:05:40 → *-*-* 08:05:40
-                     05:40 → *-*-* 05:40:00
-    Sat,Sun 12-05 08:05:40 → Sat,Sun *-12-05 08:05:40
-          Sat,Sun 08:05:40 → Sat,Sun *-*-* 08:05:40
-          2003-03-05 05:40 → 2003-03-05 05:40:00
-05:40:23.4200004/3.1700005 → 05:40:23.420000/3.170001
-            2003-02..04-05 → 2003-02,03,04-05 00:00:00
-      2003-03-05 05:40 UTC → 2003-03-05 05:40:00 UTC
-                2003-03-05 → 2003-03-05 00:00:00
-                     03-05 → *-03-05 00:00:00
-                    hourly → *-*-* *:00:00
-                     daily → *-*-* 00:00:00
-                 daily UTC → *-*-* 00:00:00 UTC
-                   monthly → *-*-01 00:00:00
-                    weekly → Mon *-*-* 00:00:00
-                    yearly → *-01-01 00:00:00
-                  annually → *-01-01 00:00:00
-                     *:2/3 → *-*-* *:02/3:00</programlisting>
+                *-*-7 0:0:0 → *-*-07 00:00:00
+                      10-15 → *-10-15 00:00:00
+        monday *-12-* 17:00 → Mon *-12-* 17:00:00
+  Mon,Fri *-*-3,1,2 *:30:45 → Mon,Fri *-*-01,02,03 *:30:45
+       12,14,13,12:20,10,30 → *-*-* 12,13,14:10,20,30:00
+            12..14:10,20,30 → *-*-* 12,13,14:10,20,30:00
+  mon,fri *-1/2-1,3 *:30:45 → Mon,Fri *-01/2-01,03 *:30:45
+             03-05 08:05:40 → *-03-05 08:05:40
+                   08:05:40 → *-*-* 08:05:40
+                      05:40 → *-*-* 05:40:00
+     Sat,Sun 12-05 08:05:40 → Sat,Sun *-12-05 08:05:40
+           Sat,Sun 08:05:40 → Sat,Sun *-*-* 08:05:40
+           2003-03-05 05:40 → 2003-03-05 05:40:00
+ 05:40:23.4200004/3.1700005 → 05:40:23.420000/3.170001
+             2003-02..04-05 → 2003-02,03,04-05 00:00:00
+       2003-03-05 05:40 UTC → 2003-03-05 05:40:00 UTC
+                 2003-03-05 → 2003-03-05 00:00:00
+                      03-05 → *-03-05 00:00:00
+                     hourly → *-*-* *:00:00
+                      daily → *-*-* 00:00:00
+                  daily UTC → *-*-* 00:00:00 UTC
+                    monthly → *-*-01 00:00:00
+                     weekly → Mon *-*-* 00:00:00
+                     yearly → *-01-01 00:00:00
+                   annually → *-01-01 00:00:00
+                      *:2/3 → *-*-* *:02/3:00</programlisting>
 
       <para>Calendar events are used by timer units, see
       <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>
diff --git a/src/basic/calendarspec.c b/src/basic/calendarspec.c
index e4cfab364e..fda293fcb9 100644
--- a/src/basic/calendarspec.c
+++ b/src/basic/calendarspec.c
@@ -302,6 +302,17 @@ int calendar_spec_to_string(const CalendarSpec *c, char **p) {
 
         if (c->utc)
                 fputs(" UTC", f);
+        else if (IN_SET(c->dst, 0, 1)) {
+
+                /* If daylight saving is explicitly on or off, let's show the used timezone. */
+
+                tzset();
+
+                if (!isempty(tzname[c->dst])) {
+                        fputc(' ', f);
+                        fputs(tzname[c->dst], f);
+                }
+        }
 
         r = fflush_and_check(f);
         if (r < 0) {
@@ -747,9 +758,9 @@ fail:
 }
 
 int calendar_spec_from_string(const char *p, CalendarSpec **spec) {
+        const char *utc;
         CalendarSpec *c;
         int r;
-        const char *utc;
 
         assert(p);
         assert(spec);
@@ -760,11 +771,39 @@ int calendar_spec_from_string(const char *p, CalendarSpec **spec) {
         c = new0(CalendarSpec, 1);
         if (!c)
                 return -ENOMEM;
+        c->dst = -1;
 
         utc = endswith_no_case(p, " UTC");
         if (utc) {
                 c->utc = true;
                 p = strndupa(p, utc - p);
+        } else {
+                const char *e = NULL;
+                int j;
+
+                tzset();
+
+                /* Check if the local timezone was specified? */
+                for (j = 0; j <= 1; j++) {
+                        if (isempty(tzname[j]))
+                                continue;
+
+                        e = endswith_no_case(p, tzname[j]);
+                        if(!e)
+                                continue;
+                        if (e == p)
+                                continue;
+                        if (e[-1] != ' ')
+                                continue;
+
+                        break;
+                }
+
+                /* Found one of the two timezones specified? */
+                if (IN_SET(j, 0, 1)) {
+                        p = strndupa(p, e - p - 1);
+                        c->dst = j;
+                }
         }
 
         if (strcaseeq(p, "minutely")) {
@@ -1017,7 +1056,7 @@ static int find_next(const CalendarSpec *spec, struct tm *tm, usec_t *usec) {
         for (;;) {
                 /* Normalize the current date */
                 (void) mktime_or_timegm(&c, spec->utc);
-                c.tm_isdst = -1;
+                c.tm_isdst = spec->dst;
 
                 c.tm_year += 1900;
                 r = find_matching_component(spec->year, &c.tm_year);
diff --git a/src/basic/calendarspec.h b/src/basic/calendarspec.h
index f6472c1244..c6087228fd 100644
--- a/src/basic/calendarspec.h
+++ b/src/basic/calendarspec.h
@@ -37,6 +37,7 @@ typedef struct CalendarComponent {
 typedef struct CalendarSpec {
         int weekdays_bits;
         bool utc;
+        int dst;
 
         CalendarComponent *year;
         CalendarComponent *month;
diff --git a/src/basic/time-util.c b/src/basic/time-util.c
index 24e681bf85..0ef1f6393e 100644
--- a/src/basic/time-util.c
+++ b/src/basic/time-util.c
@@ -254,32 +254,95 @@ struct timeval *timeval_store(struct timeval *tv, usec_t u) {
         return tv;
 }
 
-static char *format_timestamp_internal(char *buf, size_t l, usec_t t,
-                                       bool utc, bool us) {
+static char *format_timestamp_internal(
+                char *buf,
+                size_t l,
+                usec_t t,
+                bool utc,
+                bool us) {
+
+        /* The weekdays in non-localized (English) form. We use this instead of the localized form, so that our
+         * generated timestamps may be parsed with parse_timestamp(), and always read the same. */
+        static const char * const weekdays[] = {
+                [0] = "Sun",
+                [1] = "Mon",
+                [2] = "Tue",
+                [3] = "Wed",
+                [4] = "Thu",
+                [5] = "Fri",
+                [6] = "Sat",
+        };
+
         struct tm tm;
         time_t sec;
-        int k;
+        size_t n;
 
         assert(buf);
-        assert(l > 0);
 
+        if (l <
+            3 +                  /* week day */
+            1 + 10 +             /* space and date */
+            1 + 8 +              /* space and time */
+            (us ? 1 + 6 : 0) +   /* "." and microsecond part */
+            1 + 1 +              /* space and shortest possible zone */
+            1)
+                return NULL; /* Not enough space even for the shortest form. */
         if (t <= 0 || t == USEC_INFINITY)
+                return NULL; /* Timestamp is unset */
+
+        sec = (time_t) (t / USEC_PER_SEC); /* Round down */
+        if ((usec_t) sec != (t / USEC_PER_SEC))
+                return NULL; /* overflow? */
+
+        if (!localtime_or_gmtime_r(&sec, &tm, utc))
                 return NULL;
 
-        sec = (time_t) (t / USEC_PER_SEC);
-        localtime_or_gmtime_r(&sec, &tm, utc);
+        /* Start with the week day */
+        assert((size_t) tm.tm_wday < ELEMENTSOF(weekdays));
+        memcpy(buf, weekdays[tm.tm_wday], 4);
 
-        if (us)
-                k = strftime(buf, l, "%a %Y-%m-%d %H:%M:%S", &tm);
-        else
-                k = strftime(buf, l, "%a %Y-%m-%d %H:%M:%S %Z", &tm);
+        /* Add the main components */
+        if (strftime(buf + 3, l - 3, " %Y-%m-%d %H:%M:%S", &tm) <= 0)
+                return NULL; /* Doesn't fit */
 
-        if (k <= 0)
-                return NULL;
+        /* Append the microseconds part, if that's requested */
         if (us) {
-                snprintf(buf + strlen(buf), l - strlen(buf), ".%06llu", (unsigned long long) (t % USEC_PER_SEC));
-                if (strftime(buf + strlen(buf), l - strlen(buf), " %Z", &tm) <= 0)
-                        return NULL;
+                n = strlen(buf);
+                if (n + 8 > l)
+                        return NULL; /* Microseconds part doesn't fit. */
+
+                sprintf(buf + n, ".%06llu", (unsigned long long) (t % USEC_PER_SEC));
+        }
+
+        /* Append the timezone */
+        n = strlen(buf);
+        if (utc) {
+                /* If this is UTC then let's explicitly use the "UTC" string here, because gmtime_r() normally uses the
+                 * obsolete "GMT" instead. */
+                if (n + 5 > l)
+                        return NULL; /* "UTC" doesn't fit. */
+
+                strcpy(buf + n, " UTC");
+
+        } else if (!isempty(tm.tm_zone)) {
+                size_t tn;
+
+                /* An explicit timezone is specified, let's use it, if it fits */
+                tn = strlen(tm.tm_zone);
+                if (n + 1 + tn + 1 > l) {
+                        /* The full time zone does not fit in. Yuck. */
+
+                        if (n + 1 + _POSIX_TZNAME_MAX + 1 > l)
+                                return NULL; /* Not even enough space for the POSIX minimum (of 6)? In that case, complain that it doesn't fit */
+
+                        /* So the time zone doesn't fit in fully, but the caller passed enough space for the POSIX
+                         * minimum time zone length. In this case suppress the timezone entirely, in order not to dump
+                         * an overly long, hard to read string on the user. This should be safe, because the user will
+                         * assume the local timezone anyway if none is shown. And so does parse_timestamp(). */
+                } else {
+                        buf[n++] = ' ';
+                        strcpy(buf + n, tm.tm_zone);
+                }
         }
 
         return buf;
@@ -539,12 +602,11 @@ int parse_timestamp(const char *t, usec_t *usec) {
                 { "Sat",       6 },
         };
 
-        const char *k;
-        const char *utc;
+        const char *k, *utc, *tzn = NULL;
         struct tm tm, copy;
         time_t x;
         usec_t x_usec, plus = 0, minus = 0, ret;
-        int r, weekday = -1;
+        int r, weekday = -1, dst = -1;
         unsigned i;
 
         /*
@@ -609,15 +671,55 @@ int parse_timestamp(const char *t, usec_t *usec) {
                 goto finish;
         }
 
+        /* See if the timestamp is suffixed with UTC */
         utc = endswith_no_case(t, " UTC");
         if (utc)
                 t = strndupa(t, utc - t);
+        else {
+                const char *e = NULL;
+                int j;
+
+                tzset();
+
+                /* See if the timestamp is suffixed by either the DST or non-DST local timezone. Note that we only
+                 * support the local timezones here, nothing else. Not because we wouldn't want to, but simply because
+                 * there are no nice APIs available to cover this. By accepting the local time zone strings, we make
+                 * sure that all timestamps written by format_timestamp() can be parsed correctly, even though we don't
+                 * support arbitrary timezone specifications.  */
+
+                for (j = 0; j <= 1; j++) {
+
+                        if (isempty(tzname[j]))
+                                continue;
+
+                        e = endswith_no_case(t, tzname[j]);
+                        if (!e)
+                                continue;
+                        if (e == t)
+                                continue;
+                        if (e[-1] != ' ')
+                                continue;
+
+                        break;
+                }
 
-        x = ret / USEC_PER_SEC;
+                if (IN_SET(j, 0, 1)) {
+                        /* Found one of the two timezones specified. */
+                        t = strndupa(t, e - t - 1);
+                        dst = j;
+                        tzn = tzname[j];
+                }
+        }
+
+        x = (time_t) (ret / USEC_PER_SEC);
         x_usec = 0;
 
-        assert_se(localtime_or_gmtime_r(&x, &tm, utc));
-        tm.tm_isdst = -1;
+        if (!localtime_or_gmtime_r(&x, &tm, utc))
+                return -EINVAL;
+
+        tm.tm_isdst = dst;
+        if (tzn)
+                tm.tm_zone = tzn;
 
         if (streq(t, "today")) {
                 tm.tm_sec = tm.tm_min = tm.tm_hour = 0;
@@ -634,7 +736,6 @@ int parse_timestamp(const char *t, usec_t *usec) {
                 goto from_tm;
         }
 
-
         for (i = 0; i < ELEMENTSOF(day_nr); i++) {
                 size_t skip;
 
@@ -727,7 +828,6 @@ parse_usec:
                         return -EINVAL;
 
                 x_usec = add;
-
         }
 
 from_tm:
diff --git a/src/basic/time-util.h b/src/basic/time-util.h
index 1b058f0e49..99be5ce6ee 100644
--- a/src/basic/time-util.h
+++ b/src/basic/time-util.h
@@ -68,7 +68,9 @@ typedef struct triple_timestamp {
 #define USEC_PER_YEAR ((usec_t) (31557600ULL*USEC_PER_SEC))
 #define NSEC_PER_YEAR ((nsec_t) (31557600ULL*NSEC_PER_SEC))
 
-#define FORMAT_TIMESTAMP_MAX ((4*4+1)+11+9+4+1) /* weekdays can be unicode */
+/* We assume a maximum timezone length of 6. TZNAME_MAX is not defined on Linux, but glibc internally initializes this
+ * to 6. Let's rely on that. */
+#define FORMAT_TIMESTAMP_MAX (3+1+10+1+8+1+6+1+6+1)
 #define FORMAT_TIMESTAMP_WIDTH 28 /* when outputting, assume this width */
 #define FORMAT_TIMESTAMP_RELATIVE_MAX 256
 #define FORMAT_TIMESPAN_MAX 64
diff --git a/src/test/test-calendarspec.c b/src/test/test-calendarspec.c
index 4a2b93de59..57d9da4855 100644
--- a/src/test/test-calendarspec.c
+++ b/src/test/test-calendarspec.c
@@ -88,6 +88,27 @@ static void test_next(const char *input, const char *new_tz, usec_t after, usec_
         tzset();
 }
 
+static void test_timestamp(void) {
+        char buf[FORMAT_TIMESTAMP_MAX];
+        _cleanup_free_ char *t = NULL;
+        CalendarSpec *c;
+        usec_t x, y;
+
+        /* Ensure that a timestamp is also a valid calendar specification. Convert forth and back */
+
+        x = now(CLOCK_REALTIME);
+
+        assert_se(format_timestamp_us(buf, sizeof(buf), x));
+        printf("%s\n", buf);
+        assert_se(calendar_spec_from_string(buf, &c) >= 0);
+        assert_se(calendar_spec_to_string(c, &t) >= 0);
+        calendar_spec_free(c);
+        printf("%s\n", t);
+
+        assert_se(parse_timestamp(t, &y) >= 0);
+        assert_se(y == x);
+}
+
 int main(int argc, char* argv[]) {
         CalendarSpec *c;
 
@@ -155,5 +176,7 @@ int main(int argc, char* argv[]) {
         assert_se(calendar_spec_from_string("00:00:00/0.00000001", &c) < 0);
         assert_se(calendar_spec_from_string("00:00:00.0..00.9", &c) < 0);
 
+        test_timestamp();
+
         return 0;
 }
diff --git a/src/test/test-time.c b/src/test/test-time.c
index ee7d55c5ab..7078a0374d 100644
--- a/src/test/test-time.c
+++ b/src/test/test-time.c
@@ -19,6 +19,7 @@
 
 #include "strv.h"
 #include "time-util.h"
+#include "random-util.h"
 
 static void test_parse_sec(void) {
         usec_t u;
@@ -201,6 +202,48 @@ static void test_usec_sub(void) {
         assert_se(usec_sub(USEC_INFINITY, 5) == USEC_INFINITY);
 }
 
+static void test_format_timestamp(void) {
+        unsigned i;
+
+        for (i = 0; i < 100; i++) {
+                char buf[MAX(FORMAT_TIMESTAMP_MAX, FORMAT_TIMESPAN_MAX)];
+                usec_t x, y;
+
+                random_bytes(&x, sizeof(x));
+                x = x % (2147483600 * USEC_PER_SEC) + 1;
+
+                assert_se(format_timestamp(buf, sizeof(buf), x));
+                log_info("%s", buf);
+                assert_se(parse_timestamp(buf, &y) >= 0);
+                assert_se(x / USEC_PER_SEC == y / USEC_PER_SEC);
+
+                assert_se(format_timestamp_utc(buf, sizeof(buf), x));
+                log_info("%s", buf);
+                assert_se(parse_timestamp(buf, &y) >= 0);
+                assert_se(x / USEC_PER_SEC == y / USEC_PER_SEC);
+
+                assert_se(format_timestamp_us(buf, sizeof(buf), x));
+                log_info("%s", buf);
+                assert_se(parse_timestamp(buf, &y) >= 0);
+                assert_se(x == y);
+
+                assert_se(format_timestamp_us_utc(buf, sizeof(buf), x));
+                log_info("%s", buf);
+                assert_se(parse_timestamp(buf, &y) >= 0);
+                assert_se(x == y);
+
+                assert_se(format_timestamp_relative(buf, sizeof(buf), x));
+                log_info("%s", buf);
+                assert_se(parse_timestamp(buf, &y) >= 0);
+
+                /* The two calls above will run with a slightly different local time. Make sure we are in the same
+                 * range however, but give enough leeway that this is unlikely to explode. And of course,
+                 * format_timestamp_relative() scales the accuracy with the distance from the current time up to one
+                 * month, cover for that too. */
+                assert_se(y > x ? y - x : x - y <= USEC_PER_MONTH + USEC_PER_DAY);
+        }
+}
+
 int main(int argc, char *argv[]) {
         uintmax_t x;
 
@@ -214,6 +257,7 @@ int main(int argc, char *argv[]) {
         test_get_timezones();
         test_usec_add();
         test_usec_sub();
+        test_format_timestamp();
 
         /* Ensure time_t is signed */
         assert_cc((time_t) -1 < (time_t) 1);