From type/DateTime

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method posix(Bool:D: $ignore-timezone = False --> Int:D)

Returns the date and time as a POSIX/UNIX timestamp (integral seconds since the POSIX epoch, 1970-01-01T00:00:00Z).

If $ignore-timezone is True, the DateTime object will be treated as if the time zone offset is zero.

say DateTime.new('2015-12-24T12:23:00Z').posix;       # OUTPUT: «1450959780␤»