:sig
String time_format_utc(String format = "", u64 timestamp = 0)

:params
format : formatting string specifying the date format
timestamp : optional timestamp value, defaults to current time
return value : a formatted date

:see
>time
>time_format_relative

:content
Formats a timestamp in the GMT/UTC timezone.

This formatter is based on the Linux `date` command and supports the same core formatting tokens plus UCE's relative-time delta tokens.

Supported format sequences:

```text
%%     a literal %
%a     locale's abbreviated weekday name (e.g., Sun)
%A     locale's full weekday name (e.g., Sunday)
%b     locale's abbreviated month name (e.g., Jan)
%B     locale's full month name (e.g., January)
%c     locale's date and time (e.g., Thu Mar  3 23:05:25 2005)
%C     century; like %Y, except omit last two digits (e.g., 20)
%d     day of month (e.g., 01)
%D     date; same as %m/%d/%y
%e     day of month, space padded; same as %_d
%F     full date; like %+4Y-%m-%d
%g     last two digits of year of ISO week number (see %G)
%G     year of ISO week number (see %V); normally useful only with %V
%h     same as %b
%H     hour (00..23)
%I     hour (01..12)
%j     day of year (001..366)
%k     hour, space padded ( 0..23); same as %_H
%l     hour, space padded ( 1..12); same as %_I
%m     month (01..12)
%M     minute (00..59)
%n     a newline
%N     nanoseconds (000000000..999999999)
%p     locale's equivalent of either AM or PM; blank if not known
%P     like %p, but lower case
%q     quarter of year (1..4)
%r     locale's 12-hour clock time (e.g., 11:11:04 PM)
%R     24-hour hour and minute; same as %H:%M
%s     seconds since 1970-01-01 00:00:00 UTC
%S     second (00..60)
%t     a tab
%T     time; same as %H:%M:%S
%u     day of week (1..7); 1 is Monday
%U     week number of year, with Sunday as first day of week (00..53)
%V     ISO week number, with Monday as first day of week (01..53)
%w     day of week (0..6); 0 is Sunday
%W     week number of year, with Monday as first day of week (00..53)
%x     locale's date representation (e.g., 12/31/99)
%X     locale's time representation (e.g., 23:13:48)
%y     last two digits of year (00..99)
%Y     year
%z     +hhmm numeric time zone (e.g., -0400)
%:z    +hh:mm numeric time zone (e.g., -04:00)
%::z   +hh:mm:ss numeric time zone (e.g., -04:00:00)
%:::z  numeric time zone with : to necessary precision (e.g., -04, +05:30)
%Z     alphabetic time zone abbreviation (e.g., EDT)
%deltaS total elapsed seconds between the timestamp and now
%deltaM total elapsed minutes between the timestamp and now
%deltaH total elapsed hours between the timestamp and now
%deltad total elapsed days between the timestamp and now
%deltam total elapsed 30-day months between the timestamp and now
%deltaY total elapsed 365-day years between the timestamp and now
```

Padding and case flags may follow `%`:

```text
-   do not pad the field
_   pad with spaces
0   pad with zeros
+   pad with zeros, and put '+' before future years with >4 digits
^   use upper case if possible
#   use opposite case if possible
```

Related:

- PHP: `gmdate()` and UTC `DateTime` formatting
- JavaScript / Node.js: `Date#toISOString()` and UTC formatting helpers
