Adding months keeps the same day of the month. When that day does not exist, the last day of the month is used: one month after 31 January is 28 February (or 29 in a leap year).
Working days skip Saturdays and Sundays, which is how notice periods and delivery times are often counted. Public holidays are not skipped.
Common questions
What date is 90 days from today?
Leave the start date as today, choose Add, enter 90 and Days.
Is 30 days the same as one month?
No. Months have 28 to 31 days, so adding one month and adding 30 days can give different dates.