+6 or +3 is irrelevant to the scheduler when they're setting the meeting time: they want it to be 6pm regardless of any time zone changes in the future. So they deliberately do not store the UTC offset with that time. They only store the geographic location.
Then when the meeting time is approaching, the software can figure out what time zone (and UTC offset) applies, after taking account any time zone changes that have taken place in the intervening years, and appropriately set reminder notifications.
If they were to store "X seconds from the time this message was sent", then any time zone change would likely make it not fall at 6pm on the target day, so that's not what they'd want.
Then when the meeting time is approaching, the software can figure out what time zone (and UTC offset) applies, after taking account any time zone changes that have taken place in the intervening years, and appropriately set reminder notifications.
If they were to store "X seconds from the time this message was sent", then any time zone change would likely make it not fall at 6pm on the target day, so that's not what they'd want.