Expiry Date Calculator

Expiry math is integer date arithmetic anchored to the production date. The forward path computes expiry = production + shelf_life_days by adding whole days (n * 86400000) to Date.UTC(y, m, d), or calendar-exact months/years with last-day clamping (31 Jan + 1 month = 28/29 Feb; 31 Jan + 30 days = 2 Mar 2025 because February has 28 days). The reverse path computes production = expiry - shelf_life_days the same way. To avoid DST drift around midnight local time, the page anchors arithmetic to UTC via Date.UTC(y, m, d) and adds whole milliseconds (n * 86400000), because Date.setDate in local time crosses DST boundaries with a one-hour offset that can flip the visible date on March and November transitions in the US, October in the EU.

Units are added directly on the calendar: months clamp to the last day of the target month and years count as 12 months, matching the calendar-exact rule used by regulators. The Codex Alimentarius General Standard for the Labelling of Prepackaged Foods (CXS 1-1985, revised) and China's GB 7718-2025 specify shelf life from the date of manufacture or packaging; pharmaceutical expiry follows ICH Q1A (R2) stability testing guidelines and rounds the printed expiry to the last day of the month (so a drug manufactured 2024-08-15 with an 18-month shelf life is labelled 2026-02-28, not 2026-02-15). The remaining-days counter is (expiry_utc - now_utc) / 86400000 rounded with Math.floor for already-elapsed days and Math.ceil for time-until-expiry, and the freshness band is a thresholded comparison: expired when remaining < 0, expiring-soon when 0 ≤ remaining ≤ 30, fresh otherwise.

• Date arithmetic: parse to Y-M-D, anchor to Date.UTC(y, m, d) and add whole days (n * 86400000); months/years are added calendar-exactly with last-day clamping. Never use local-time Date.setDate - it crosses DST boundaries and can flip the visible date.
• Unit handling: months are added calendar-exactly with last-day clamping (31 Jan + 1 month = 28/29 Feb), years as 12 months; pharmaceutical labels follow ICH Q1A (R2) and print the last day of the expiry month.
• Remaining days = (expiry_utc - now_utc) / 86400000, Math.ceil for time until expiry / Math.floor for elapsed days; thresholds: remaining < 0 → expired, 0 ≤ remaining ≤ 30 → expiring soon, otherwise fresh.
• Regulatory anchors vary: food shelf life follows Codex CXS 1-1985 / GB 7718-2025 (from production or packaging); cosmetics use PAO (Period After Opening) starting at first use; medicines use ICH Q1A stability dating.
• After-opening shelf life is independent of the printed expiry and is shorter (e.g. opened olive oil ~3 months despite 24-month sealed shelf life); track the open date separately.
• Browser clock skew (a misconfigured system date) shifts the freshness band; the page reads new Date() rather than a remote time source, so it inherits any client-side clock error.