I had the same headaches with this through native Bubble so I built my own plugin to solve this. Since then Bubble has made some changes to how you can manipulate time zones (I think), so it may be possible in native Bubble, but I am sure it is still a pain.
So although you asked for free plugins, I have to recommend my paid TopCal plugin, as I know it works well and can save you days of development, testing, failing, forum posts and then retrying. If you have questions about it let me know.
Link to a way you can test it out
Link to Forum Post about it
Link to Example Editor and Videos for how to use