Suppose you want to calculate a date that is 91 days from the current date. You can use the following formula:

\[=TODAY()\]

Alternatively, you can use the EOMONTH function to calculate the last day of the month, 3 months from the current date:

The Excel 91 formula is not a specific formula but rather a colloquialism used to describe a combination of Excel functions that calculate a date that is a certain number of days before or after a specified date. The number 91 in the formula represents a period of 91 days, which is equivalent to 3 months.

\[=EOMONTH(TODAY(), 1)\]

This function is useful for calculating intervals between the current date and a specific date.

\[=EOMONTH(TODAY(), 3)\]

For example: