send datepart as parameter from a table to DATEADD function in sql server

I Have to Update City.Date in City table and I have columns in the table like Interval and Period in the City table.

Interval column contains values like yy,ww,dd,qq,etc and Period column contains values like 1,2,3.

I am trying to update City.Date like this:

UPDATE City 
SET City.date = DATEADD(City.Interval, City.Period, City.date)
WHERE CityId = 13

It is getting error like:

City.Interval is not recognized DATEADD option.

How can I update City.Date using City.Interval, City.Period and City.date?