How to Convert Timestamps in JavaScript: Unix, ISO 8601, and More

Timestamp handling is one of those things every JavaScript developer does constantly but rarely thinks about systematically — until a bug caused by a missing * 1000 or an unexpected timezone shift hits production. This guide is a complete reference for how to convert timestamps in JavaScript: Unix seconds to ISO 8601, the Date object API, timezone formatting with Intl.DateTimeFormat, and when to reach for a library like date-fns or dayjs. For quick conversions between Unix timestamps and human-readable dates, use our Timestamp Converter — no code required. The JavaScript Date Object The built-in Date object is JavaScript's foundation for time. All timestamps internally are stored as milliseconds since the Unix epoch (January 1, 1970, 00:00:00 UTC): {codeDateBasics} The most common gotcha: the Date constructor's month parameter is 0-indexed (January = 0, December = 11). This is a legacy quirk you cannot avoid — just memorize it. Unix Timestamps: Seconds vs. Milliseconds Unix timestamps