If you want to calculate the estimated delivery date, take the mating date and add 338 days or 11 months to get the foaling date. In case you want to calculate the full range of possible foaling days follow next steps: for the earliest possible foaling take mating date and add 331 days. For the latest possible foaling take mating date and add 346 days. In most cases it will be impossible to accurately predict the foaling date so take those numbers more as a guidance and pay attention to your mare's behavior and make sure she has everything she needs.