Thanks so much in advance for your help. I'm thinking a macro is what
I need, but I'm not sure.

A3 is the current date.
L9 is the current number of mutual fund units that a client owns
G9 is the amount of money the client invests into the fund every month
on the 5th.
M9 is the current price of the mutual fund.

I need a trigger that will update L9 on the 5th of each month.