jQuery UI: datepicker - how to get a field date then insert a maniulated date elsewhere?

jQuery UI: datepicker - how to get a field date then insert a maniulated date elsewhere?

I am trying to do a few things I simply cannot find information about.
- when setting a date with datepicker (jquery UI), I need to immediately modify the datepicker entry to the last day of the chosen month, and then, set the following field in my document to that date + 1 day.
I do not have a clue how to achieve this...
Any help would be greatly apopreciated.