I encountered several challenges for date formulas and date functions.
Date formulas I found interesting are shown here.
How to calculate monthly fractions of date differences correctly you can see here.
Fwim - Determine the first or i-th weekday of a month.
Lwim - Determine last or last but (i-1)-th weekday in a month.
Weekday_dom - Lists all days between two dates with a given weekday and a given day of month
Count_hours - [A VBA solution] If you need to count specified hours for specified weekdays and holidays
Add_hours - Same as Count_Hours but you add the hours to a date and time