How to calculate an end date to be 13 days after the beginning date that user chooses?

[farash12]

I am trying to create a time sheet and need to know how to do calculated date fields. I have a BeginDate field and EndDate field. I want the end date to populate automatically with 13 days after the beginning date that was selected. I will use formcalc or javascript. I am a beginner so please be detailed. Thank you!