Bit pointless for 1 hour, seems a bit harsh for you to then owe them the 2.5 hours.
Is it too late for you to take a holiday - then you won't forfit the 2.5 hours? Maybe that is what they are hoping for, to force you to take a days holiday?
Seems weird.
Slight tangent, my mum has worked most Christmas's for the past 10 years - all day 8 -5pm. She is a nurse so it is expected, she said she would rather people with younger children got the day at home with their families.
This year, she has booked the day off, to spend with her grandchild. One woman in her unit is kicking off massively, it isn't fair apparently (this person took last year off) because her children are young and she believes mum should do a split shift with her. Makes my blood boil - if you train as a Dr/Nurse/Police/Fireman or similar, you know that your shift patterns are not regular and will fall over Christmas at some point. It is all about give and take surely? I think 99% of those in that position, understand that but there is always a minority who feel they are hard done to.
Bearing in mind, when I was younger my mum often worked Christmas day shifts - so I am more than aware of the impact on young families. Grrrrr. Sorry, sore point, mum is really upset by her colleagues attitude and I am angry on her behalf!