DATEADD (Transact-SQL)

This function adds a specified number value (as a signed integer) to a specified datepart of an input date value, and then returns that modified value.

See Date and Time Data Types and Functions (Transact-SQL) for an overview of all Transact-SQL date and time data types and functions.

DATEADD (datepart , number , date ) 

datepart
The part of date to which DATEADD adds an integer number. This table lists all valid datepart arguments.

day的缩写是dd或d

DATEADD (Transact-SQL)

原文：https://www.cnblogs.com/chucklu/p/11782410.html