From type/DateTime

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method in-timezone(DateTime:D: Int(Cool) $timezone = 0 --> DateTime:D)

Returns a DateTime object for the same time, but in the specified $timezone, which is the offset in seconds from GMT.

say DateTime.new('2015-12-24T12:23:00Z').in-timezone(3600 + 1800); # OUTPUT: «2015-12-24T13:53:00+0130␤»

Per RFC 7164, leap seconds do not respect local time and always occur at the end of the UTC day:

say DateTime.new: '2017-01-01T00:59:60+01:00'
# OUTPUT: «2017-01-01T00:59:60+01:00␤»