isTokenExpired function
Whether token has expired, treating anything within offsetSeconds of its
expiry as already gone.
Implementation
bool isTokenExpired(String token, [num offsetSeconds = 0]) {
final expiry = _expiryOf(token);
final nowMillis =
DateTime.now().millisecondsSinceEpoch + (offsetSeconds * 1000);
return expiry.millisecondsSinceEpoch < nowMillis;
}