Add or subtract days, weeks, months or years to a date.
Set the start date to today, choose "Days" as the unit, enter 90, and make sure the operation is set to add — the result date, day of year, ISO week number and weekday all update automatically.
It's the week-of-the-year number defined by ISO 8601, where week 1 is the week containing the year's first Thursday. It's the standard used on most calendars and planners outside the US.
Yes — switch the operation to subtract to go backwards from the start date by the same days/weeks/months/years units.