libc/ctime.3: timegm() is in C23.

See also:	D53951, https://wiki.freebsd.org/C23
MFC after:	1 week

lib/libc/stdtime/ctime.3

@@ -27,7 +27,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd September 23, 2025
+.Dd May 14, 2026
 .Dt CTIME 3
 .Os
 .Sh NAME
@@ -354,8 +354,9 @@ table).
 .Pp
 The
 .Fn timegm
-function is not specified by any standard; its function cannot be
-completely emulated using the standard functions described above.
+function is expected to conform to
+.St -isoC-2023
+with the same constraint.
 .Sh HISTORY
 This manual page is derived from
 the time package contributed to Berkeley by