Add or Subtract Days from a Date
Updated: May 2026
Adding or subtracting a fixed number of days, weeks, months or years from a known date is a routine calculation for payment terms, shipping windows, warranty expiry dates and legal notice periods. The answer is never as simple as mental arithmetic because months have different lengths, leap years shift the calendar, and business-day rules skip weekends.
Free · No upload · In your browser
Payment terms: Net-30, Net-45, Net-60
Net payment terms define how many calendar days a buyer has to pay an invoice after the invoice date. Net-30 means payment is due 30 calendar days from the invoice date. An invoice issued on March 5 is due April 4 — not April 5, because 30 days after March 5 is April 4 (March has 31 days: 31 − 5 = 26 remaining in March; 30 − 26 = 4 days into April).
A common mistake is to add 30 to the day-of-month, which gives April 5 (wrong), or to advance one calendar month, which gives April 5 (also wrong for most of the year). The only correct approach is to count the exact number of calendar days forward.
- Net-15 — 15 calendar days, common for small or time-sensitive transactions
- Net-30 — 30 calendar days, the most common term in commercial invoicing
- Net-45 — 45 calendar days, common in retail supply chains
- Net-60 — 60 calendar days, typical in large-enterprise procurement
- Net-90 — 90 calendar days, common in some manufacturing and government contracts
Shipping and delivery due dates
When an order ships today and the carrier promises delivery in 5 business days, the due date skips Saturday and Sunday. An item shipped Thursday evening may not arrive until the following Wednesday (Thursday → Friday = 1 business day, then Monday–Wednesday = 3 more, total = 4; add one more for the Thursday partial = 5). The business-days-add function in the Flowfiles calculator handles this correctly.
Reverse logistics — return windows, exchange eligibility and warranty claim cutoffs — use the same logic. A 30-day return window starting from the purchase date ends exactly 30 calendar days later, regardless of weekends. The clock does not stop for holidays.
For shipping, the "ship date" is often Day 0 and delivery counts from Day 1. So "3 business days" from a Thursday ship date means delivery on Tuesday (Friday = Day 1, Monday = Day 2, Tuesday = Day 3). Enable "Skip weekends" in the Add / Subtract tab to compute this automatically.
Warranty and guarantee expiry dates
Consumer warranties are expressed in days, months or years from the purchase date. A 12-month warranty purchased March 15 expires March 14 the following year (or March 15, depending on whether the manufacturer uses "from date" or "through date" language). A 90-day warranty purchased June 1 expires August 30 — use the add-days function to verify before filing a claim.
Extended warranties and service plans are sold in month or year increments. Knowing the exact expiry date — not just the approximate month — is necessary for scheduling preventive service visits or negotiating renewals before coverage lapses.
Legal notice periods
Many legal rights and obligations have notice periods: rental notice to vacate, contract termination notices, appeal windows, and court filing deadlines. These periods are counted in calendar days from the date of service, receipt or triggering event. Missing a notice window by one day can forfeit the right entirely or expose a party to liability.
Subtract the notice period from the target effective date to find the latest date by which notice must be given. Use the Subtract function: enter the effective date, subtract the required notice period in days, and the result is the latest service date.
Frequently asked questions
How do I add 30 days to a date?
In the Add / Subtract tab, enter your start date, set amount to 30, unit to Days, and operation to Add. The result date appears immediately. Enable "Skip weekends" to count only business days and land on a weekday.
What is the difference between adding 1 month and adding 30 days?
Adding 1 month advances to the same calendar date in the following month — March 15 + 1 month = April 15. Adding 30 days always moves exactly 30 calendar days — March 15 + 30 days = April 14. These differ whenever the starting month has more or fewer than 30 days.
How do I calculate a Net-30 due date?
Enter the invoice date as the start date, set amount to 30, unit to Days, and operation to Add. The result is the exact payment due date. Net-30 always uses calendar days, not business days.
Can I add business days only?
Yes. Enable the "Skip weekends" option in the Add / Subtract tab when unit is set to Days. The calculator will advance through weekdays only, skipping Saturdays and Sundays.