I have an assignment where I have to create a birthday list. One of the tasks is to add a countdown til someone's birthday, I have the formula as:

=DAYS360($D$200,I3)
However how do I calculate it so if some's birthday past I can recalculate it so it is not negative.

Like if someone's birthday past it will say -105 days, where I want to add 365 to any negative amounts.

You probably will need to write an IF statement.

That is, IF the answer is negative, add 365; otherwise, calculate regularly.

Yeah but I can't figure out how to do that, how do I do an IF function within the calculation?

Go to the help part of Excel and type in if statement. It will give you instructions.

To calculate a countdown until someone's birthday that updates to a positive value if the birthday has already passed, you can use a combination of the IF and IFERROR functions in Excel.

First, use the following formula to calculate the initial countdown until someone's birthday:
=DAYS360($D$200, I3)

Assuming that cell I3 contains the date of someone's birthday.

Next, wrap the formula inside an IF function to check if the result is negative. If it is negative, add 365 to the result, else return the original result:

=IF(DAYS360($D$200, I3) < 0, DAYS360($D$200, I3) + 365, DAYS360($D$200, I3))

By adding 365 to the negative result, you effectively reset the countdown for the next year.

Please note that the cell $D$200 is assumed to contain the current date in your spreadsheet. Update the cell reference accordingly based on your actual spreadsheet.

This revised formula will give you the countdown until the birthday, adjusting it to be positive if the birthday has already passed.