Why is the year in this ISO timestamp not 2019?

For a simple app I'm using to test a devops pipeline I'm outputting the start time of a build to the homepage. On my development machine the year of the ISO-8601 timestamp I expect, 2019, is printed (specifically, "2019-09-12T20:11:00.000Z"). When the same codebase is built using AWS CodeBuild the ISO-8601 timestamp looks like "+051668-02-09T08:09:32.000Z". What is "+051668"? I presume it's the year; My best guess is it's the year represented as a different calendar. Thoughts?

AWS CodeBuild sets this environment variable for every build (CODEBUILD_START_TIME). I'm building with their latest, default, Ubuntu container (v2.0).

Most of the google hits show CODEBUILD_START_TIME being represented as the number of milliseconds since 1970. I suspect somewhere in your code you are treating that as if it were seconds.

If you convert +051668-02-09T08:09:32.000Z to seconds you get 1568320819772. If you divide+round that by 1000 you get 1568320819 then convert that back to a human readable time you get September 12, 2019 8:40:19.