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Timestamp is in seconds so yes it's 60*30 == 1800 seconds for half an hour.
If you're going to cut DST it's not going to change (I don't think) - the timestamp itself will still be the number of seconds since 00:00 01/01/1970) even if the actual time is something else i.e. the timestamp doesn't suddently jump 3600 seconds at 1am or whatever