Returns a copy of this
ZonedDateTime with the specified number of months subtracted.
This operates on the local time-line,
subtracting months to the local date-time.
This is then converted back to a
ZonedDateTime, using the zone ID
to obtain the offset.
When converting back to
ZonedDateTime, if the local date-time is in an overlap,
then the offset will be retained if possible, otherwise the earlier offset will be used.
If in a gap, the local date-time will be adjusted forward by the length of the gap.
This instance is immutable and unaffected by this method call.
|the months to subtract, may be negative|
ZonedDateTime based on this date-time with the months subtracted, not null
|if the result exceeds the supported date range|