How to manage summer and winter datetimes in php

I have a site where I need to store a lot of events with a start and end date. So I store the startDate and endDate at GMT time in the database.

However, when France switches to winter time, we end up with not GMT +2 but GMT +1. No problem for converting a DateTime to the correct time… But how do you manage dates that were set before the switch to winter time and are scheduled for the future (once winter time has been changed)? My whole schedule is off by an hour.

I think my problem is fairly trivial, but I’m having trouble understanding where I went wrong. What is the right way to do? How to avoid this type problem?

Thanks for your precious help!

Store datetime in GMT timezone in database