updateTask() sending wrong dates to backend

I am consistently getting the wrong dates when I try to make changes to task dates in a custom editor.
(This pic is from developer tools)
(This is from the network tab)
I have no idea why I am getting the wrong start_date, any help is greatly appreciated.

I ended up making my own ajax request and then loading the json back in once it finished.

Probably, the issue is related to the Gantt configuration, but it is hard to suggest what might be wrong as I don’t see your code.
Could you please provide the simplified demo where I can reproduce the issue locally?