I believe you are not setting both elements in the X data list to the new date value.
Try this app and see that two elements in the X data list are adjusted so that the trace end date is extended:
https://anvil.works/build#clone:YADLNHPUVOYIB6MW=3AEORTXIIQJJ7FRGA2ET6YVM
You should be able to adapt this concept to your specific needs I believe.