I am using Google's php api client. I am running through the quickstart guide for service accounts. I followed the steps perfectly (as far as I can tell). I am running into the following error:

{
   "error": "invalid_grant",
   "error_description": "Invalid JWT: Token must be a short-lived token (60 minutes) and in a reasonable timeframe. Check your iat and exp values and use a clock with skew to account for clock differences between systems."
}

From what I have read the most common problem with this error is if the system time is wrong. I have triple checked that my timezone and date and time are synced with the atomic clock. I used php set timezone function to set my timezone to match my computer, but I continue to get the error. I am looking into the other part of the message that mentions the iat and exp settings, and haven't gotten anywhere yet.

Does anyone have any ideas of how I can get past this?

Invalid_grant error has two common causes:

1. Your server’s clock is not in sync with NTP.

   Solution: Check the server time. If it's incorrect, fix it.

2. The refresh token limit has been exceeded.

   Solution: Nothing you can do - they can't have more refresh tokens in use.

   Applications can request multiple refresh tokens. For example, this is useful in situations where a user wants to install an application on multiple machines. In this case, two refresh tokens are required, one for each installation. When the number of refresh tokens exceeds the limit, older tokens become invalid. If the application attempts to use an invalidated refresh token, an invalid_grant error response is returned.

   The limit for each unique pair of OAuth 2.0 client and is 50 refresh tokens (note that this limit is subject to change). If the application continues to request refresh tokens for the same Client/Account pair, once the 26th token is issued, the 1st refresh token that was previously issued will become invalid. The 27th requested refresh token would invalidate the 2nd previously issued token and so on.

Update

Applications in testing phase have their refresh tokens revoked after seven days. Go to Google cloud console for your project and under oauth2 set it to production.