Working with Dates is there a clear tutorial on how to convert UTC to Local and Local to UTC?

Trying to get my head around the best way to convert date and time from UTC to local time and visa versa.

Spending many hours trying to find a simple solution to this and I am not more confused than when I started!

Quite a simple problem.

I store dates in UTC in the database. I need to submit in local time (converted to save in UTC) and then display in local time (converted from saved UTC)

Is there a simple and clear tutorial on how to do this somewhere?

Thanks!