90-Day Probation Periods: How to Count Them Correctly

The "90-day probation period" is standard in US employment, and similar constructs exist worldwide (up to 6 months in China and Germany, typically 3 months in the UK). Miscounting it has real consequences: benefits eligibility, termination rights, and notice requirements often hinge on the exact end date.

90 days is not 3 months

Three calendar months from June 1 is September 1 — that's 92 days. From February 1 it's May 1 — just 89 or 90 days. If a contract says 90 days, count exactly 90 calendar days; if it says 3 months, land on the same day-of-month three months later. The two diverge by up to three days, and disputes have turned on exactly this difference.

Which day is day one?

The default legal convention is that the start date is day zero — the first day of employment is not the first day counted; the period ends at the close of the 90th day after it. But some contracts state "including the start date." Read the wording; when drafting, spell out the end date explicitly ("probation ends at the close of business on October 2, 2026") to remove all ambiguity.

Calendar days or business days?

Probation periods are almost always calendar days — weekends and holidays count. Business days show up in the surrounding mechanics instead: "written notice within 5 business days", "benefits begin the first business day after probation." Both types can appear in one clause, so match each number to its unit.

For employers

• Compute the exact end date at hire and put it in the offer letter.
• If an extension is allowed, diary the decision at least two weeks before the end date — extending after the period lapses is often invalid.
• Remember that in many jurisdictions statutory protections apply from day one regardless of probation.

For employees

• Confirm your exact probation end date in writing during week one.
• Benefits with waiting periods (health cover, 401(k) matching) may start on a different clock — "first of the month after 90 days" is common and can add up to 30 extra days.

To get the exact date, put your start date into the days-between calculator, or check 90 days from today if you're starting now.