userdb: add birthDate field to JSON user records

Add a birthDate field to the JSON user record, stored internally as a
struct tm with INT_MIN/negative sentinels for unset fields. The field
is serialized as a YYYY-MM-DD string in JSON and validated via
parse_birth_date(), which shares its core logic with
parse_calendar_date() through a new parse_calendar_date_full()
function.

For birth dates, timegm() is called directly (rather than
mktime_or_timegm_usec) to support pre-epoch dates. The wday field is
used to distinguish timegm() failure from a valid (time_t) -1 return.

birthDate is excluded from user_record_self_modifiable_fields(), so
only administrators can set or change it via homectl. The field
remains in the regular (non-privileged) JSON section, keeping it
readable by the user and applications.

src/basic/time-util.c

@@ -1893,9 +1893,8 @@ TimestampStyle timestamp_style_from_string(const char *s) {
         return t;
 }
 
-int parse_calendar_date(const char *s, usec_t *ret) {
+int parse_calendar_date_full(const char *s, bool allow_pre_epoch, usec_t *ret_usec, struct tm *ret_tm) {
         struct tm parsed_tm = {}, copy_tm;
-        usec_t usec;
         const char *k;
         int r;
 
@@ -1906,9 +1905,22 @@ int parse_calendar_date(const char *s, usec_t *ret) {
                 return -EINVAL;
 
         copy_tm = parsed_tm;
-        r = mktime_or_timegm_usec(&copy_tm, /* utc= */ true, &usec);
-        if (r < 0)
-                return r;
+
+        usec_t usec = USEC_INFINITY;
+
+        if (allow_pre_epoch) {
+                /* For birth dates we use timegm() directly since we need to accept pre-epoch dates.
+                 * timegm() returns (time_t) -1 both on error and for one second before the epoch.
+                 * Initialize wday to -1 beforehand: if it remains -1 after the call, it's a genuine
+                 * error; if timegm() changed it, the date was successfully normalized. */
+                copy_tm.tm_wday = -1;
+                if (timegm(&copy_tm) == (time_t) -1 && copy_tm.tm_wday == -1)
+                        return -EINVAL;
+        } else {
+                r = mktime_or_timegm_usec(&copy_tm, /* utc= */ true, &usec);
+                if (r < 0)
+                        return r;
+        }
 
         /* Refuse non-normalized dates, e.g. Feb 30 */
         if (copy_tm.tm_mday != parsed_tm.tm_mday ||
@@ -1916,8 +1928,15 @@ int parse_calendar_date(const char *s, usec_t *ret) {
             copy_tm.tm_year != parsed_tm.tm_year)
                 return -EINVAL;
 
-        if (ret)
-                *ret = usec;
+        if (ret_usec)
+                *ret_usec = usec;
+        if (ret_tm) {
+                /* Reset to unset, then fill in only the date fields we parsed and validated */
+                *ret_tm = BIRTH_DATE_UNSET;
+                ret_tm->tm_mday = parsed_tm.tm_mday;
+                ret_tm->tm_mon = parsed_tm.tm_mon;
+                ret_tm->tm_year = parsed_tm.tm_year;
+        }
 
         return 0;
 }

src/basic/time-util.h

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include <limits.h>
 #include <time.h>
 
 #include "basic-forward.h"
@@ -180,7 +181,23 @@ const char* etc_localtime(void);
 
 int mktime_or_timegm_usec(struct tm *tm, bool utc, usec_t *ret);
 int localtime_or_gmtime_usec(usec_t t, bool utc, struct tm *ret);
-int parse_calendar_date(const char *s, usec_t *ret);
+
+int parse_calendar_date_full(const char *s, bool allow_pre_epoch, usec_t *ret_usec, struct tm *ret_tm);
+
+static inline int parse_calendar_date(const char *s, usec_t *ret) {
+        return parse_calendar_date_full(s, /* allow_pre_epoch= */ false, ret, NULL);
+}
+
+#define BIRTH_DATE_UNSET                        \
+        (const struct tm) {                     \
+                .tm_year = INT_MIN,             \
+        }
+
+#define BIRTH_DATE_IS_SET(tm) ((tm).tm_year != INT_MIN)
+
+static inline int parse_birth_date(const char *s, struct tm *ret) {
+        return parse_calendar_date_full(s, /* allow_pre_epoch= */ true, NULL, ret);
+}
 
 uint32_t usec_to_jiffies(usec_t usec);
 usec_t jiffies_to_usec(uint32_t jiffies);