I am designing a family budgeting sheet that works by entering income and expenditure each day of the year. The first cell , let's say it is B1, contains the amount let's say, $800 representing rent per fortnight. That would be placed beside the date it is to be paid for the whole year by using =B$1 and projecting it down month by month by dragging.

This means that I can change the number at B1 (if the rent goes up) and they all change. If the rent goes to $850 per fortnight, I can change the number at B1 to 850 to reset what we have to pay. Trouble is, it alters ALL the figures for every fortnight and I just want it to alter the future ones yet to be paid.
At A column, all the dates are listed. How do I write an if/then type function/formula that could be added to =B$1 to make it not change if the date is historical?

whew...hope this makes sense

Bruce