interpreting coin base api historical price data
This seems to correspond to the ISO 8601 datetime format, specifically the complete date plus hours, minutes and seconds with time zone designation format.

YYYY-MM-DDThh:mm:ssTZD (e.g. 1997-07-16T19:20:30+01:00)


The TZD hereby stands for time zone designator, which is a ±hh:mm offset from UTC* . 

Accordingly, "-05:00" would represent Eastern Standard Time.

* UTC (Coordinated Universal Time) is also known as GMT (Greenwich Mean Time), i.e. the time in London's time zone.