TallyDays

How we calculate

These are the rules every calculator on TallyDays follows. Each result page repeats the rules that apply to it under "How this is calculated".

Which days are counted

• Counts start the day after the start date and include the end date. From Monday to Wednesday is 2 days.
• "Today" is the current date in Eastern Time (America/New_York). The date a page counted from is shown at the top of the page.
• When an interval runs backward, the count is shown as a positive number of days before today.
• Calculators that let you include or exclude the start and end dates say which choice was used.

Weeks, months and years

• A week is 7 calendar days. "12 weeks + 3 days" means 87 days.
• Months are calendar months, never 30-day blocks. One month after January 31 is February 28 (February 29 in a leap year).
• Several months are added in one step from the starting date, so the result never drifts at month ends.
• February 29 only exists in leap years: years divisible by 4, except century years not divisible by 400. A countdown to February 29 always targets a real leap day.

Business days and federal holidays

• Business days are Monday through Friday, minus the 11 annual U.S. federal holidays set by 5 U.S.C. 6103.
• When a holiday falls on a Saturday, most federal employees get the Friday before off. When it falls on a Sunday, they get the Monday after off. Business-day counts skip that observed day off.
• Holiday countdowns always target the actual holiday date. The observed day off is shown separately.
• State holidays, company closures and one-time closures (such as a presidential executive order) are not included.
• Inauguration Day is not treated as a nationwide holiday, because it applies only to some federal employees in the Washington, D.C. area.

Week numbers

Week numbers follow ISO 8601: weeks start on Monday, and week 1 is the week with the year's first Thursday. Some years have a week 53; most have 52. Near New Year, a date can belong to a week of the previous or next year.

Ages

With an exact birthday, age is counted in whole years, months and days. With only a birth year, age is one of two numbers, depending on whether the birthday has happened yet this year. People born on February 29 are treated as having their birthday on February 28 in other years. This is a calendar calculation, not a legal determination of age.

Event dates

Each holiday and observance page shows how its date is set: the same date every year, a yearly rule such as "fourth Thursday in November", or a date that depends on an announcement. We do not guess dates that have not been announced.

Sources

• U.S. Office of Personnel Management: federal holidays
• OPM: holidays observed "in lieu of" a weekend date
• 5 U.S.C. 6103: the federal holiday statute

How the math is checked

The calculations are covered by more than 1,400 automated tests, including leap days, month ends, week 53 and OPM's published holiday schedules. If you find a result that looks wrong, please let us know.