If you're going to make a field be required based on a client
side event (checking a check box), then you have to use javascript.
Since the stolen date field may be omitted if the checkbox is
left empty, you wont want the date field to be required.
Once the user attempts to submit the form, you will need a
function to run to validate if the date field has been entered
corresponding to the field with the checkbox. If the date field has
not been filled out, then dont submit the form and throw an alert..
If a value exists for the date field while the checkbox is checked,
then submit the form.
If you're unfamiliar with js, then you will have to run all
of your validation on the server side, and notify the user of the
errors after they have submitted the form.