FIRUser decode of dates is incorrect.

[jbb1003]

[REQUIRED] Step 2: Describe your environment

• Xcode version: 9.3.1
• Firebase SDK version: 4.33.0
• Firebase Component: Auth
• Component version: 5.0.0

[REQUIRED] Step 3: Describe the problem

The date fields of ID tokens are incorrectly decoded in current epoch.
See [FIRUser parseIDToken:error:]

NSDate *expDate =
      [NSDate dateWithTimeIntervalSinceNow:[tokenPayloadDictionary[@"exp"] doubleValue]];

They should be decoded in UNIX epoch - see https://firebase.google.com/docs/auth/admin/verify-id-tokens

exp | Expiration time | Must be in the future. The time is measured in seconds since the UNIX epoch.

Since we are now in 2018 and epoch is 1970, we end up ~48 years in the future.

Steps to reproduce:

Get an ID token with [FIRUser getIDTokenWithCompletion:]
Observe that in parseIDToken the expiration date (and other dates) ends up in 2066.

[ryanwilson]

@jbb1003 Thanks for reporting this! I'll try to reproduce locally.