MAN DATE(1)

  %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:
  %:::z  numeric time zone with : to necessary precision (e.g., -04, +05:30)
  %Z     alphabetic time zone abbreviation (e.g., EDT)

Both %z and %Z work but none of variants with : does.