Excel Question – Sum by date when date is not constant

Can anyone please help or knows Excel function for the following. I have 24 periods, now I would like to sum them up just by changing date in one field, so for example if right now is August I would like to sum up only numbers from January through August, next month, when I enter September, I want formula to pick up September amount as well, I tried writting nested if statements but it has limitations, and I also don’t want to go into SUM formula and change every month to reflect it.
If anyone know the solution I would really appreciate it.