Google 客户端无效的 JWT:令牌必须是短期令牌

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."
}

据我所知，此错误最常见的问题是系统时间是否错误.我已经三次检查我的时区、日期和时间是否与原子钟同步.我使用 php set timezone 函数来设置我的时区以匹配我的电脑，但我仍然收到错误消息.我正在查看提到 iat 和 exp 设置的消息的另一部分，但还没有找到任何地方.