Adds or subtracts to the start date the number of working days specified and returns the resulting date. When adding or subtracting days to the start date, Saturdays and Sundays are not counted as days.

WORKDAY(Date, Days)

Date: the date to which working days are to be added or subtracted.

Days: the number of days to add or subtract.

WORKDAY(DATE(1,1,2017), 100) returns 5/19/2017