Twice last week, a developer wanted to convert the existing datetime values in a database to UTC. The datetime values were the default for the server, which was the US/Pacific time zone, which is subject to Daylight Saving Time changes. Both developers for both applications wanted to convert all the times to UTC, so there would not be any changes due to Daylight Saving Time, and asked me for an easy query to know which times should be changed by adding 7 hours and which times should have 8 hours added to them.
The good news is that MySQL has some built-in functionality to make this easier. You may know about the CONVERT_TZ() function in MySQL, and that you can use it in a query to convert times like this: