Comment by semiquaver

  > and I don't record the timezone information on the date field

Very few databases actually make it possible to preserve timezone in a timestamp column. Typically the db either has no concept of time zone for stored timestamps (e.g. SQL server) or has “time zone aware” timestamp column types where the input is converted to UTC and the original zone discarded (MySQL, Postgres)

Oracle is the only DB I’m aware of that can actually round-trip nonlocal zones in its “with time zone” type.