About

Counting 30 days forward or backward from a reference date is a frequent requirement in contract law, medical follow-ups, invoice net terms, and regulatory compliance windows. A manual count fails when the range spans month boundaries of unequal length (28, 29, 30, or 31 days) or crosses a leap-year February. Errors in deadline computation carry real consequences: missed filing dates, lapsed insurance coverage, or contractual penalties. This calculator determines the exact target date by performing integer day arithmetic on the Gregorian calendar. It also decomposes the interval into business days, weekend days, and US federal holidays so you can distinguish calendar days from working days.

The tool accepts any start date between 1900 and 2200 and any offset from −9999 to 9999 days. Floating federal holidays (Memorial Day, Thanksgiving) are computed algorithmically using the Nth-weekday-of-month rule rather than a static lookup table. Limitation: the holiday set covers US federal holidays only. State-specific or international holidays are not included.

Formulas

The target date is computed by adding an integer offset n to the start date D₀:

D_target = D₀ + n days

Where n = 30 by default. Negative n counts backward.

Business days B within the range are counted by iterating each calendar day d and testing:

B = D_target∑d = D₀ f(d)

Where f(d) is a piecewise function:

{

1 if weekday(d) ∉ {0, 6} ∧ d ∉ H0 otherwise

Where H is the set of US federal holidays in the range, and weekday(d) returns 0 for Sunday, 6 for Saturday.

Floating holidays use the Nth-weekday-of-month formula. To find the k-th occurrence of weekday w in month m of year y:

day = 1 + (w − weekday(m/1/y) + 7) mod 7 + 7 × (k − 1)

For "last Monday of May" (Memorial Day), compute the last Monday by finding the first Monday of June and subtracting 7.

Reference Data

US Federal Holiday	Rule	2024 Date	2025 Date
New Year's Day	January 1	Jan 1	Jan 1
Martin Luther King Jr. Day	3rd Monday of January	Jan 15	Jan 20
Presidents' Day	3rd Monday of February	Feb 19	Feb 17
Memorial Day	Last Monday of May	May 27	May 26
Juneteenth	June 19	Jun 19	Jun 19
Independence Day	July 4	Jul 4	Jul 4
Labor Day	1st Monday of September	Sep 2	Sep 1
Columbus Day	2nd Monday of October	Oct 14	Oct 13
Veterans Day	November 11	Nov 11	Nov 11
Thanksgiving Day	4th Thursday of November	Nov 28	Nov 27
Christmas Day	December 25	Dec 25	Dec 25
Common Net Terms in Business
Net 15	Invoice due in 15 calendar days	Short-cycle vendors
Net 30	Invoice due in 30 calendar days	Industry standard
Net 45	Invoice due in 45 calendar days	Extended terms
Net 60	Invoice due in 60 calendar days	Large enterprises
Net 90	Invoice due in 90 calendar days	Government / institutional
2/10 Net 30	2% discount if paid within 10 days	Early payment incentive
Common Compliance / Medical Windows
Prescription Refill	Typically 30 days	Controlled substances
Insurance Claim	30 - 90 days	Varies by policy
Return Window	14 - 30 days	Consumer protection
Probation Period	30 - 90 days	Employment contracts

Frequently Asked Questions

No. Day 1 is the day after the start date. If you start on January 1, day 30 is January 31. This follows the standard "exclusive start" convention used in legal and financial deadline calculations. If your jurisdiction uses an inclusive count, subtract 1 from the offset.

The JavaScript Date object natively accounts for leap years. When the 30-day window crosses February 28 in a non-leap year, the engine correctly advances to March. In a leap year, February 29 is included in the count. No manual adjustment is needed.

This tool counts US federal holidays only (11 per year). Your organization may observe additional holidays (state holidays, company-specific days off, half-days, or international holidays). The weekend definition here is Saturday and Sunday. Some industries (retail, healthcare) operate on non-standard weekly schedules. Treat the result as a baseline and adjust for your specific calendar.

Yes. Entering a negative number (e.g., −30) calculates the date 30 days before the start date. All statistics (business days, weekends, holidays) are computed for the resulting interval regardless of direction.

Net 30 calendar days includes all days: weekends and holidays. Net 30 business days excludes Saturdays, Sundays, and federal holidays, resulting in roughly 42-44 calendar days. Most invoice terms use calendar days unless explicitly stated otherwise. Always verify the contract language.

Thanksgiving is the 4th Thursday of November. The algorithm finds which weekday November 1 falls on, computes the offset to the first Thursday, then adds 21 days (3 × 7) to reach the 4th occurrence. This Nth-weekday-of-month method avoids hardcoded lookup tables and works for any year in the supported range (1900-2200).