The date calculator provides an easy way to add or subtract a number of days, months and years from an original date. Although it is designed as a general purpose tool, it is often used to calculate the earliest time you can file N-400 to apply for citizenship, by subtracting 90 days from your green card's 5 or 3 year anniversary date.
Green card holders may apply for naturalization by filing Form N-400 when they meet all eligibility requirements. "Continuous residence" is one of them, meaning that you must have been a legal permanent resident (LPR) for at least five years, or three years if you obtained your green card based on a marriage to a U.S. citizen. However, if you meet all other requirements, you are allowed to apply for naturalization up to 90 calendar days before you meet the "continuous residence" requirement.
Your N-400 early filing date can be calculated using the tool above:
USCIS may reject or deny your naturalization application if you file too early. To be on the safe side, some applicants choose to wait a few days after they become eligible to submit their application. Also, it is often referred to as "filing for citizenship 3 months before meeting the 5-year residence requirement," which is technically incorrect: it is up to 90 calendar days.