Expiry Date Calculator

Expiry math is integer date arithmetic anchored to the production date. The forward path computes expiry = production + shelf_life_days using Date.setDate(date.getDate() + n), which handles month and year rollover natively (e.g. 2025-01-31 + 30 = 2025-03-02 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. Shelf-life units are normalized to days before the addition: 1 month is treated as 30 days and 1 year as 365 days, matching the heuristic used on most consumer packaging math but not 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. The persistent watch list lives in window.localStorage under a single namespaced key, serialized via JSON.stringify, which most browsers cap at roughly 5-10 MB per origin (Chrome ~10 MB, Safari ~5 MB). When the quota is exceeded the browser throws a DOMException with name 'QuotaExceededError', and the page must fall back to in-memory state because writes after the throw are silently dropped. localStorage is also synchronous and same-origin, so the entries persist across page reloads but never leave the device. The browser Notification API (which the reminder uses) requires Notification.requestPermission(), is granted per origin, and on Chrome on Windows ≥ 86 actually surfaces through the OS notification centre rather than the browser itself.

• Date arithmetic: Date.setDate(d.getDate() + n) handles month/year rollover natively, but in local time it crosses DST boundaries and can flip the date; Date.UTC + integer ms is the safe path.
• Unit normalization: 1 month ≈ 30 days, 1 year ≈ 365 days - simple but not calendar-exact; pharmaceutical labels follow ICH Q1A (R2) and round to the last day of the expiry month.
• Remaining days = Math.floor((expiry_utc - now_utc) / 86400000); 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.
• localStorage quota is ~5-10 MB per origin and throws QuotaExceededError (DOMException) when full; writes after the throw are silently dropped, so wrap setItem in try/catch and surface a user warning.
• 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.