Since LO version 7.5 it finally works to create a custom property (File/Properties/Custom Properties) of type Duration – in previous versions the type was reset to Text when leaving the Properties dialog.
So my naive assumption was that I could go ahead now and calculate a date from a start date and a duration. Only so far I was unable to find anything that worked. The documentation for LO writer does not even have anything on custom properties of type duration, let alone an example for its usage or calculations with it. The Wiki, too, does not have anything on it. And any attempts to reference the custom properties in a formula failed as well.
Does anybody have an idea how to do it or a pointer where to look – other than the source code?