What a Unix timestamp represents
A Unix timestamp counts elapsed time from 1970-01-01 00:00:00 UTC. Most systems use seconds, while browser and JavaScript timestamps commonly use milliseconds.
Automatic unit detection
Numeric values below 100 billion in magnitude are treated as seconds; larger values are treated as milliseconds. ISO 8601 dates can also be entered directly.
Time zones
The timestamp itself represents one instant. UTC and local output differ only in how that instant is displayed.