Python: Adding 3 weeks to any date

I need help with a program.
How do I add 3 weeks (21 days) to any given date when the user can control the date?

The user will enter the date YYYY-MM-DD.

Below I'm trying to locate the hyphen and make sure there is only 2. This is what I have so far but all it does is repeat itself, can someone tell me where I went wrong ?:

date = raw_input("Enter date: ")
i = 0
while i <= len(date):
    if date[i] != "-":
    i = i + 1
print date

Now I'm picking out year, month, day. Is there an easier way to do this cause I need to account for the change months etc ?

year = date[0:4]
month = date[5:7]
day = date[9:11]

thanks