Imagine you have users all around the world and you've got logs of their activities. Someone asks "do users spend more time on X in the morning or afternoon"? I

Imagine you have users all around the world and you've got logs of their activities. Someone asks "do users spend more time on X in the morning or afternoon"? If you only recorded absolute times and threw away their local time then you can't answer that question. It doesn't matter whether you record UTC or local time as long as you record their time zone, you can always derive one from the other. This is an implementation detail that should be abstracted by any decent time library.