F# / .NET: Gotcha's
This is another annoying "feature" in .NET. For some reasons,
TimeSpan.ToString with a custom format string does all sorts of wonky stuff, particularly it will give you the "Invalid Format String" error on any unescaped character that isn't a valid format specifier.
Basically, take this list: Custom TimeSpan Format Strings and treat it literally.
Want to add a space between days and hours/minutes/seconds? Escape it:
d\ hh\:mm\:ss. Yes, even spaces.