How to convert time::Tm to u64 for JWT expiration date - time

Question

I'm using the jwt crate and I want to set the expiration date in the Claims struct. The exp field in Registered took a Option<u64>.
I can retrieve the current date and add 1 day to it by doing:
let mut timer = time::now();
timer = timer + Duration::days(1);
but I don't see how to convert this time::Tm to a u64.

Answer 1

The exp field is of "NumericDate" type, which according to RFC 7519 is "number of seconds from 1970-01-01T00:00:00Z UTC until the specified UTC date/time, ignoring leap seconds."
This description is the same as the to_timespec method, which "Convert time to the seconds from January 1, 1970" in the Tm's current timezone*.
Thus:
let mut timer = time::now_utc();
timer = timer + Duration::days(1);
token.claims.reg.exp = Some(timer.to_timespec().sec as u64);
(Note that while time + duration always return UTC time as of v0.1.36, it is arguably a defect that could be fixed in a future. To be forward-compatible, I used now_utc() instead of now().)
(*: to_timespec basically calls gmtime() on POSIX and the POSIX standard ignores leap seconds. On Windows it converts the structure to a FILETIME which again ignores leap seconds. So to_timespec is safe to use if you really care about the 27-second difference.)
If you are using std::time::SystemTime, the same can be obtained using
let mut timer = SystemTime::now();
timer += Duration::from_secs(86400);
token.claims.reg.exp = Some(timer.duration_since(UNIX_EPOCH).unwrap().as_secs());