setting an set number of 41 and having it reset to 0 once 41 is reached

Ok so i have a spreadsheet that i need to have a set number of 41, and what i want to have happen when it reaches 41 is to reset back to 0. but the key is to having the doc use a calander to include weekends and weekdays, if a varible is set to have duty during the week not to have duty on the weekends, and vice versa so there is no overlapping. now the formula I am trying to use is

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Now i have color coding on the cells that are at or near 21, turning them from while shading to red and when the cel reaches 41 being a sharp red, but i cant get it to reset back to 0 at the begining of the month so people dont get screwed. Also how do i set a flag next to the cell that is at 21, paramaters are 0-15 green, 16-31 yellow, 32-41 red at cell A1

If anyone could please help me with this.. I will love you long time, and many thanks in advance for helping.

[INDENT] I'm thinking like a Liberty Basic programmer on this and the function that i do believe it would be is a for loop with a reset condition with in the function:

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then putting a break in there and an excption to catch it at 21 then reset it back to 0. but excell is a little different. >.>