daily overtime

let me explain: daily I have to do 7.5 hours but with flexibility that can lead me to do 6 or 10 hours. if I do more than 7.5 it becomes a credit that can be spent to fill the days where I only do 6 hours. so that my days are always 7.5. At the end of the year my credit/debit must be at 0. Currently my overtime counter is incrementing well, but does not regularize the shorter days.