daily fixed hours minimum

Although I like the idea of tracking my total hours worked, I have an greement with my client that I work a minimum of 8 hours per day and this is all I invoice for, even if I work slightly longer. Similar to a previous query of ‘time in lieu’, or another where it will calculate the minimum ours worked accumulating over a pay cycle…. For example, say I am required to work a minimum of 8 hours per day (40 hours per 5 day week, but I do say 9 hours per day, it would advise that I have reached my minimum hours of 40 for that week, and calculate the total of the additional hours, in this case 5 hours of extra time, that I do not invoice for, but a great record as evidence of the additional hours put in if ever needed .