A PERIOD(TIMESTAMP(n) [WITH TIME ZONE]) value can be cast as PERIOD(TIMESTAMP[(n)] [WITH TIME ZONE]) using the CAST function.
The result date and time portions are set to the corresponding source date and time portions. If the target type specifies WITH TIME ZONE and the source also contains time zones, the time zone displacements in the source are copied to the corresponding result elements. If the source does not contain time zones, the current session time zone displacement is copied to both result elements except if the source ending bound is the maximum value, the time zone for the result ending bound is +00:00.
If the target precision is higher than the source precision, trailing zeros are added in the fractional seconds. If the target precision is lower than the source precision, an error is reported.