Excel Hours Tracker

[Loopylee2020]

Hi all, new to the forum nice to meet you all.

I am in need of help. I am trying to do an hours tracker where I start off with a pot of 100 hours and then depending on how long I stay at work (I work annualised hours) I can plus or minus them hours worked off my 100 hours
For instance I started August with 75 hours on the first Monday I stay half an hour after my finish time of 10pm so 30 minutes would come off my 75hours if I finished at 9:50 pm I would add on 10 minutes. I would like the same thing for every day of the year

Any help would be appreciated I can get it to work if I stay after work but if I finish early I can’t minus the hours to put them back on

Thanks
Lee Watson