I calculated mine to be in September, originally. Later, I had to move it to May, and I had a friend who was very cross that I was technically no longer a Virgo.

But it's a bit nonsense, really. The day you were born doesn't change, just the day you use to count your age. I've known some people who get confused over their birthday due to the year changing. It's just that time itself is convoluted to start with, and measuring it only convolutes things further.

I suppose the deeper question is, do you want to attempt an accurate measure, or do you want to eat cake?