I'm pretty sure it was around April 4th last year, so She's at the 340 day mark now. I don't know if she's ever had a baby before, so no past time frames to go on. The foaling calculator I did said anywhere from March 1 -March 15. I realize it can go way past that and still be normal. How nerve wrecking!
Best wishes for a safe delivery and healthy foal! Good luck on catching the delivery. I missed my last mare foaling by 30 minutes. Checked on her, everything normal and 30 minutes later, had a buckskin baby on the ground. You just never know!
Just for future reference, you can have pregnancies aborted early in horses with just a shot of lute (in this case I would have given the shot within the week of covering). It's a lot less than having another horse and can be bought right from your vet.