URL: https://www.ietf.org/rfc/rfc3339.txt
Written in 2002 by The Internet Society, this is a brilliant historical document all about "defin[ing] a date and time format for use in Internet protocols that is a profile of the ISO 8601 standard for representation of dates and times using the Gregorian calendar."
Time is a very hard thing to work with, and this was a super cool read.