Adding 90 Days to a Date: Contracts, Notice Periods, and Deadlines
How to correctly add 90 days to a date for contracts, notice periods, invoice due dates, and legal deadlines, and the calendar-vs-business-day trap that catches people out.
"90 days from today" shows up everywhere: net-90 invoices, probation periods, notice clauses, visa windows, return policies. It sounds obvious, but the wrong interpretation can cost you a payment or void a contract.
Calendar days or business days?
Read the clause carefully. "Within 90 days" almost always means calendar days, including weekends and holidays. "Within 90 business days" is roughly 18 weeks, which is nearly double. If the document just says "days," courts and tax authorities in most jurisdictions treat that as calendar days.
Do you include the start date?
The common-law default is to exclude the day the clock starts and include the last day. So a notice served on March 1 with a 90-day period expires on May 30, not May 31. Some statutes flip this — check the exact wording ("from and including" vs "after").
Weekend and holiday rollover
- If day 90 lands on a weekend or public holiday, most contracts roll the deadline to the next business day.
- Tax and court deadlines almost always roll forward. Payment terms usually don't unless the contract says so.
- Never roll backward to the previous Friday — that shortens the period and is rarely what the drafter meant.
Quick reference from today
For a fast answer, our Add Days Calculator handles calendar-day math (including leap years) and shows the resulting weekday. For working-day counts like "90 business days from today," switch to the Business Days Calculator, which skips weekends and optional holiday lists.
Spreadsheet equivalents
In Excel and Google Sheets, =TODAY()+90 gives you 90 calendar days out. For business days use =WORKDAY(TODAY(), 90, holidays), where holidays is an optional range of dates to skip. Both formulas assume the start date is excluded, matching the common-law convention above.
Working backward from a deadline
If you know the deadline and need to find the "must-notify-by" date, subtract instead. The Date Difference Calculator confirms exactly how many days sit between any two dates, which is handy for double-checking a 30/60/90-day trigger before you send.