Extensive time period calculations and individual calendar periods.
Online Documentation: https://www.codeproject.com/Articles/168662/Time-Period-Library-for-NET
English Document: https://github.com/Giannoudis/TimePeriodLibrary/blob/master/docu/TimePeriod_en.doc
German Document: https://github.com/Giannoudis/TimePeriodLibrary/blob/master/docu/TimePeriod_de.doc