Event cycle and 'On Submit' - timing problems


I have a form where a set of fields is populated by selecting from a dropdown at the top (very standard pattern). The top dropdown is set to run a JS query (on select run…) which sets one of the following fields based on another of the following fields - the first has a date, populated correctly by the selection in the top dropdown, and the second must be set to be 25 years after the first, hence a small js script triggered by the selection of the top dropdown.

So the problem is that there is no event to use to trigger the setting of the second date field based on the first being populated. I have to run it using the ‘on select’ event of the dropdown at the top of the page, and this is unfortunately run prior to the first date field being populated, not after it. This means the script does not pick up the new value of the first date field, but the previous one, which is wrong.

Can I access the event cycle of the first date field (a datetimepicker control) in any way to run my JS?