add dateTimeHelpers (#2344)

* Add MHZC02

Arduino Library for MHZ series CO2 sensors.

* Add dateTimeHelpers

A collection of date and time helper functions

Author: RobTillaart

repositories.txt

@@ -3512,6 +3512,7 @@ https://github.com/RobTillaart/DAC8550
 https://github.com/RobTillaart/DAC8551
 https://github.com/RobTillaart/DAC8552
 https://github.com/RobTillaart/DAC8554
+https://github.com/RobTillaart/dateTimeHelpers
 https://github.com/RobTillaart/DEVFULL
 https://github.com/RobTillaart/DEVNULL
 https://github.com/RobTillaart/DEVRANDOM