Add or Subtract Days from Date

Date arithmetic involves adding or subtracting time units (days, weeks, months, years) to a reference date to calculate a target date. While adding days is straightforward, adding months and years introduces complexity due to varying month lengths and leap years. For example, adding one month to January 31 could result in February 28 or March 2, depending on the convention used. Date arithmetic is fundamental to contracts, project planning, medical scheduling, and any deadline-driven process.

Deadlines drive the modern world. Legal notice periods, warranty expirations, subscription renewals, payment due dates, and project milestones all depend on accurate date calculations. A 90-day return policy starting March 15 ends on June 13 — not an intuitive calculation when crossing months of different lengths. Financial instruments use specific date conventions (30/360, actual/actual) that affect payment schedules. Getting dates wrong can mean missed deadlines, expired warranties, or contractual disputes.

Adding days is the simplest operation — each day adds exactly one calendar day, automatically handling month and year boundaries. Adding weeks multiplies by 7 days. Adding months preserves the day-of-month when possible (March 15 + 1 month = April 15) but clamps to the month end when needed (January 31 + 1 month = February 28/29). Adding years preserves the month and day, with February 29 rolling to February 28 in non-leap years. Subtracting works identically in reverse.

Always specify whether you are counting calendar days or business days. When communicating dates across timezones, include the timezone or use ISO 8601 format (YYYY-MM-DD). For critical deadlines, add a buffer day to account for time-of-day differences. Verify date calculations across month and year boundaries, especially around February. For recurring events, recalculate from the original date rather than the last occurrence to avoid drift.