Days and weeks are added exactly. Months and years keep the same day of the month where possible; if the target month is shorter (for example January 31 plus one month), the result is the last day of that month. Use a negative amount or the direction menu to go back in time.
Leave the start date on today and the amount at 90 days; the result shows the date and weekday.
No. Adding 1 day to Monday gives Tuesday. To count the start date as day one, add one day less.
See also: Days Between Dates · Day of Week Calculator · Age Calculator