Sounds like the problem I was having here https://www.wix.com/code/home/forum/community-discussion/wix-code-changes-revert-after-a-few-seconds-on-page-load
It’s a bug, which will be fixed, but as a temporary workaround I may suggest you to initialize new date object from a value (note the use of ‘new’)
let myDate = new Date(itemData.start);