I wrote several RTC GPS date time libraries. The most complete library uses UTC for RTC chips and GPS. It handles time zones and daylight/summer time. It can format date time in ISO formats or other custom formats. It has all popular POSIX conversions between epoch time and local/UTC. It has a database and algorithms for common DST rules.This library is overkill for most users so I never posted it.....This is more of a problem of knowing what users need than writing code. Once you know the requirements code is easy.