Numbers are read as Unix time (since 1970-01-01T00:00:00Z). The unit is guessed from the digit count: up to 11 digits = seconds, 12–14 = milliseconds, 15–17 = microseconds, 18+ = nanoseconds. Override it with the Unit menu. Negative numbers (before 1970) and decimals work.
Text is read as a date: ISO 8601 (2024-03-10T14:30:00Z), RFC 2822 (Sun, 10 Mar 2024 14:30:00 GMT) or anything your browser's Date can parse. A date-only ISO value is UTC; a date-time without an offset is your local time.
Day of year and ISO week use your local calendar date. ISO weeks start on Monday; week 1 holds the year's first Thursday.
Click a copy button to copy one value. Shortcut: Ctrl/⌘+Enter copies the main result (ISO 8601 UTC for a number, Unix seconds for a date).