We keep dates too. Unfortunately our sows don't always keep the same dates
We have 2 sows that have always been served on the same day, and one always farrows 2-3 days later
Well, we know that now, so know when to expect them. But we have had a gilt farrow nearly a week early, and one caught us out completely: had seen her and Big Biff "at it" but she farrowed nearly 2 weeks after the predicted date, so not even another season's worth.
But keeping a diary definitely helps!