onSubmit modify variable value

Here we go......I have a form with the following fields:
<select name="completedDate_Month">
<select name="completedDate_Day">
<select name="completedDate_Year">
<select name="completedBy">
<input type="text" name="timeSpent">
<input type="hidden" name="completedDate">
<input type="hidden" name="completed" value="False">

Need two things to happen onSubmit:
1) Value of the completedDate_Month/Day/Year need to combine into "mm/dd/yyyy" format and be assigned to completedDate.

2) IF timeSpent && completedBy != "" then completed = "True".

Hope this is clear, THANKS IN ADVANCE!