Set the "iat" (Issued At) Claim.

• If no argument is used the current unix timestamp is used as the claim.
• If a number is passed as an argument it is used as the claim directly.
• If a Date instance is passed as an argument it is converted to unix timestamp and used as the claim.
• If a string is passed as an argument it is resolved to a time span, and then added to the current unix timestamp and used as the claim.

Format used for time span should be a number followed by a unit, such as "5 minutes" or "1 day".

Valid units are: "sec", "secs", "second", "seconds", "s", "minute", "minutes", "min", "mins", "m", "hour", "hours", "hr", "hrs", "h", "day", "days", "d", "week", "weeks", "w", "year", "years", "yr", "yrs", and "y". It is not possible to specify months. 365.25 days is used as an alias for a year.

If the string is suffixed with "ago", or prefixed with a "-", the resulting time span gets subtracted from the current unix timestamp. A "from now" suffix can also be used for readability when adding to the current unix timestamp.