Hi,
when Date&Time data is collected from different time zones I believe it is necessary to be able to unify the data by bringing all into the same time zone. At present it is required to extract the offset via the Extract Date&Time Fields node and then use the Date&Time Shift to do the math.
Though, I have not tested if that really works out (but am confident it does) i.e. when the offset results in the shift of the date to either be in the past or future.
Best
Mike