What happens to the unused holiday hours you didn’t take?

Hi so on the system I have 22 hours of holiday left and because of Eid I cannot take any days off for next week, so when I book a day off in April, why does it say I have 78 hours? which is when it resets I’m sure, but then where’s the 22 hours gone that I didn’t take? Does it just go away and you can’t use them after April? I always assumed it would add onto the 78 hours. Just curious, thanks in advance!