Here is a simple to use tool Verity has made that will help you to calculate your mares estimated foaling date.
It will tell you when the 1st and 2nd Scans are due as well.
We calculate our own mares on 342 days approximate gestation period for the light breed mares.
However our Shires have so far all gone over this by an average of 8 days.
You can change the number of days of gestation if you like.
Have a play!