Determining the first and last days of a quarter is a common task for accounting applications and reporting. For web applications, I have been using Day.js (https://day.js.org/) for a long time instead of the well-known moment. There is an extension for Day.js that allows you to determine the first and last days of a quarter.

SmartFormat is a is a lightweight text templating library to replace string.Format. Unlike common String.Format, with SmartFormat you can read templates from text files or another source. Here I will share the way, how I tried to write the DateTime in the format 'dd/MM/yyyy HH:mm:ss'. It was not so obvious.

As it turned out, FluentUI DatePicker for React doesn't contain a built-in German localization. If you need one, you can use my version.

In SharePoint queries you should use date in the format 'yyyy-mm-ddTHH:MM:SSZ', for example, 2019-09-30T09:00:00Z. But the input form of the SharePoint datetime controls returns the value as 'dd.mm.yyyyy', the time value you will recieve in another way.