Pick a date and time, get the
It is a special code like
The letter after the timestamp picks the format: t and T are time, d and D are date, f and F are date plus time, and R is a live relative time like 'in 3 hours' that keeps updating.
It is the Unix timestamp (seconds since 1970) of the date and time you pick, converted from your local time. The tool does this in your browser.