JavaScript Date & Timestamp Converter

Q: Why does JavaScript use milliseconds instead of seconds?

JavaScript's Date object was designed in 1995 to match Java's java.util.Date , which also uses milliseconds. The millisecond resolution was a deliberate choice to support sub-second timing for animations and UI interactions — common browser use cases that don't require a server. Most server-side systems (Unix, databases) use seconds because sub-second precision wasn't needed when those standards were set in the 1970s.

Q: What is the best way to work with dates in JavaScript?

For simple cases, the native Date object with Intl.DateTimeFormat is sufficient. For complex date arithmetic (business day calculations, timezone-aware scheduling, duration formatting), use the Temporal API (available in modern browsers as of 2024–2025) or a library like date-fns (tree-shakeable, immutable) or dayjs (Moment.js-compatible, small). Avoid Moment.js in new projects — it's in maintenance mode and not tree-shakeable.

Q: How do I store timestamps in a database from JavaScript?

For PostgreSQL, store as TIMESTAMPTZ (timestamp with time zone) and pass ISO 8601 strings: new Date().toISOString() . For MySQL, use DATETIME with UTC storage and convert on read. For flexibility and portability, many developers store Unix timestamps as BIGINT (milliseconds) — avoids timezone handling in the database entirely, at the cost of less readable values in direct SQL queries.

Convert between JavaScript Date objects, Unix timestamps, and human-readable date strings. Paste any timestamp or ISO 8601 date string to decode it instantly.

JavaScript Timestamp and Date Conversion

JavaScript's Date object uses millisecond timestamps internally. This converter translates between Unix timestamps (seconds), JS timestamps (milliseconds), ISO 8601 strings, and local date/time representations.

JS timestamp → date: Convert Date.now() output to a readable string
Unix timestamp → JS: Convert server-side second timestamps to JS milliseconds
ISO 8601 → timestamp: Parse date strings like 2024-01-15T10:30:00Z to a numeric value
Timezone display: See both UTC and local time for any input

Convert JavaScript Timestamps →

JavaScript Date Quick Reference

Current ms timestamp: Date.now() — returns milliseconds since epoch, no new needed
Current seconds timestamp: Math.floor(Date.now() / 1000)
Timestamp to Date object: new Date(msTimestamp) — pass ms directly; multiply seconds by 1000 first
Date to ISO string: new Date().toISOString() → "2024-01-15T10:30:00.000Z"
Parse ISO string: new Date("2024-01-15T10:30:00Z").getTime() → milliseconds
Format for display: new Date().toLocaleString("en-US", { timeZone: "America/New_York" })

Common JavaScript Date Pitfalls

Seconds vs. milliseconds: new Date(1700000000) gives January 20, 1970 (treating it as ms) — you want new Date(1700000000 * 1000) for a Unix second timestamp
Month is 0-indexed: new Date(2024, 0, 15) is January 15 — month 0 = January, month 11 = December
Date string parsing is implementation-dependent: new Date("2024-01-15") is parsed as UTC midnight; new Date("01/15/2024") is parsed as local time — prefer ISO 8601 with explicit timezone
DST transitions: When doing date arithmetic across DST boundaries, use UTC methods (getUTCHours, setUTCDate) to avoid off-by-one-hour bugs

Frequently Asked Questions

Why does JavaScript use milliseconds instead of seconds?

JavaScript's Date object was designed in 1995 to match Java's java.util.Date, which also uses milliseconds. The millisecond resolution was a deliberate choice to support sub-second timing for animations and UI interactions — common browser use cases that don't require a server. Most server-side systems (Unix, databases) use seconds because sub-second precision wasn't needed when those standards were set in the 1970s.

What is the best way to work with dates in JavaScript?

For simple cases, the native Date object with Intl.DateTimeFormat is sufficient. For complex date arithmetic (business day calculations, timezone-aware scheduling, duration formatting), use the Temporal API (available in modern browsers as of 2024–2025) or a library like date-fns (tree-shakeable, immutable) or dayjs (Moment.js-compatible, small). Avoid Moment.js in new projects — it's in maintenance mode and not tree-shakeable.

How do I store timestamps in a database from JavaScript?

For PostgreSQL, store as TIMESTAMPTZ (timestamp with time zone) and pass ISO 8601 strings: new Date().toISOString(). For MySQL, use DATETIME with UTC storage and convert on read. For flexibility and portability, many developers store Unix timestamps as BIGINT (milliseconds) — avoids timezone handling in the database entirely, at the cost of less readable values in direct SQL queries.