Note: I will reply here because I think this was the first topic reporting the problem about IP1 ignoring the new date when using the copy-invoice feature,
This bug has been fixed and now the new copied invoice gets the date entered in the modal window (aka popup). If you don’t want to wait for the next release v1.5.10 to enjoy the fix, apply the changes below to fix it in your running version.
Note: always make a backup copy of any file before editing it.
<label for="invoice_date_created"><?php _trans('invoice_date'); ?>: </label>
<label for="invoice_date_created_modal"><?php _trans('invoice_date'); ?>: </label>
<input name="invoice_date_created" id="invoice_date_created" class="form-control datepicker"
<input name="invoice_date_created_modal" id="invoice_date_created_modal" class="form-control datepicker"
If someone applies these changes, please report back whether these changes have fixed the problem in your case.
Now I will check whether the same problem happens with other input fields in all modal windows.