Happily since 3.6 was released you can do this directly with the GREL
value.timeSinceUnixEpochToDate()
This works directly if the timestamp is in seconds since the unix epoch. If you have other units you can specify:
value.timeSinceUnixEpochToDate('second')
value.timeSinceUnixEpochToDate('millisecond')
value.timeSinceUnixEpochToDate('microsecond')
Best wishes
Owen