How can I create a spreadsheet that keeps a running total of something that is automatically updated each day when I open it?

Eg. I have 12 eggs on 13 March, but I want it to take away 2 eggs every day from the total, so when I open the spreadsheet on 14 March it will automatically tell me I have 10 eggs, and on 15 March 8 eggs etc - rather than me updating it every day myself subtracting 2 eggs.

Surely if it can count down to a date using the "today" function, it can do this?

Thanks