🧭 JavaScript Date Fundamentals
Core concepts: UTC vs local time, ISO parsing, leap years, timezone math, and DST-safe foundations.
Production-focused date and timezone engineering
Build reliable JavaScript date/time features across locales and DST boundaries.
Practical guidance for full-stack teams working with Date, Intl, and Temporal in real-world apps. Learn strict parsing, UTC normalization, and explicit IANA timezone patterns that hold up in production.
About this guide
Dates are one of the most error-prone parts of any JavaScript codebase. The native Date object stores time as UTC internally but exposes confusingly named local-time methods, parses non-standard strings differently across browsers, and mutates in place — so the same code can show the wrong hour after a daylight saving change, count a day too many across a timezone, or break only for users in a single region.
This site is a working reference for getting it right. Every page pairs a clear explanation of the underlying model with copy-pasteable, TypeScript-first solutions: the legacy Date approach, the modern Intl and Temporal equivalent, and the edge cases — spring-forward gaps, fall-back overlaps, month-end rollovers, and leap years — that quietly cause production bugs. It is organized into three areas, from core fundamentals through locale-aware formatting to the immutable Temporal API.
🌍 Intl API & Legacy Date Patterns
Locale-aware formatting, cross-browser quirks, safe timezone detection, and migration patterns.
⚙️ Modern Temporal API
Immutable temporal logic, ZonedDateTime workflows, calendar systems, and production Temporal adoption.
Popular guides
Jump straight to the questions developers hit most often:
- Convert local time to UTC
- Days between two dates, ignoring DST
- Convert a Unix timestamp to a Date
- Format a date in a specific timezone
- Show “time ago” labels with Intl
- Migrate Moment.js to Temporal
- Add months without overflow
- Calculate age in years
- Convert a time between timezones
- Business days between two dates
- Format a date range with Intl
- Check if two date ranges overlap
New to this? Start here
If you are modernizing an existing codebase, begin with the fundamentals to get the mental model straight, then move to Intl formatting, and finally adopt Temporal for new date logic.