Google seems to be showing various calendars correctly with the right timezone offsets for when when added feeds to my Gcal.
So not sure what you mean here.
I believe the iCal specification (from a quick scan) can’t do “normal” unix timestamps.
iCal runs off these formatted strings (TimeZone/LongForm date), which according to the spec is based on ISO 8601
See page 35 - https://www.ietf.org/rfc/rfc2445.txt