Methodology and sources

This page explains how each calculator works and lists the official sources behind the rules and deadlines mentioned on the site. When a source and this site disagree, the source is right. Please let us know.

Date arithmetic

• Days from a date count calendar days. Day 1 is the day after the start date unless you choose to count the start day.
• Days between dates count the end date but not the start date, unless you include it.
• Adding months keeps the day of the month. If that day does not exist, the result is the last day of the month (the same rule as Excel’s EDATE).
• Age counts whole years, then whole months, then days. Leap-day birthdays are treated as February 28 in non-leap years.

Business days and holidays

• Business days are Monday to Friday. When selected, the 11 US federal holidays are skipped on their observed dates. Sources: 5 U.S.C. § 6103 and OPM federal holidays.
• Banks follow the Federal Reserve holiday schedule, which does not close on the Friday before a Saturday holiday.
• State, local and religious holidays are not included.

Time zones and seasons

• Time zones and daylight saving rules come from the IANA time zone database, built into your browser.
• Equinox and solstice times come from the US Naval Observatory, shown as dates in US Eastern Time.

Sources for rules mentioned on the site

• Credit card billing errors (60 days) and statement timing (21 days): CFPB, Regulation Z § 1026.13 and § 1026.5.
• Debit card and electronic transfer errors (60 days): CFPB, Regulation E § 1005.6.
• COBRA continuation coverage: US Department of Labor.
• Special enrollment periods and coverage until age 26: HealthCare.gov, special enrollment and young adults.
• EEOC charge filing deadlines (180 or 300 days): EEOC.
• Family and Medical Leave Act (12 workweeks): US Department of Labor.
• WARN Act layoff notices (60 days): US Department of Labor.
• Social Security full retirement age: Social Security Administration.
• Medicare initial enrollment: Medicare.gov.
• Early retirement plan withdrawals (age 59½): IRS Topic 557.
• Visa Waiver Program stays (90 days) and I-94 records: CBP and I-94 lookup.
• H-1B employment: USCIS.
• FTC Cooling-Off Rule (3 business days): FTC.
• Leftovers and thawing times: USDA FSIS, leftovers and safe thawing.

These summaries are general information, not legal, financial or medical advice.