If you're just trying to get a list of values for the hidden field, either make it a dropdown so you can extract the options from the select, or put the hidden date field in a collection and use the "as new rows" option so it automatically adds them.

Based on what you're describing, it sounds like you're looking at the awesomeplete list of results, which only appears when the field has focus. This list is added/removed on-demand, so it is not going to be the most stable source.

Another option is to flip it around the other way. Instead of pulling the entire list every time, use the visible date field as a parameter on the database lookup and populate a hidden field with any matches. If a match is found, use a custom validator to flag the input as invalid.