Powershell Get-date minus 616 days.

The problem i'm facing is as follows:

I need the date for 88 weeks ago (616 days),

I managed to create this string: (get-date).AddDays(-616).ToString("dd-mm-yyyy")

However the date it produces is 31-36-2013 and as we all know there are only 12 month in a year.

Is there a way to fix this?

Any suggestions would be appreciated,