So on one tab I have my colleagues enter certain values for that day and on
another tab (a month overview and a year overview) it automatically copies
the values over and then uses them in formulas to calculate data for graphs.

I can get the data to copy over if the days match, and for it to remain
blank if not. But how can I automatically set it up that once the day has
been, it saves the information it had copied over previously rather than
going blank because the dates no longer match? (Hence altering formulas and
graphs?)

Thanks in advance.