When saving a date to SharePoint item, use the browser's Time zone. Not the user profile or the site regional setting
The users of SharePoint online may change the Time zone they are in occasionally. They may travel from cost to cost for a few days then come back.
Using the time zone in profile may be wrong when calculating the UTC of the date-time entered. Also, the date-time of the "Created" field.
Since everything in "Modern" UI is Client-side based, so should the current user's Time Zone. Most computers I know, are running windows 10 with the automatic time zone.