Basic Syntax - Add Functions

Syntax:     AddDays([DateTime], 20)
Example:  AddDays([OrderDate], 20)

Add function being used in the expression

The DateTime value that will be used to add/subtract from

The amount of Days, Hours, etc. Added/Subtracted (Negative Values Subtract)

Basic Syntax - DateDiff Functions

Syntax:     DateDiffDay(StartDate, EndDate)
Example: DateDiffDay([OrderDate], [ShippedDate])

DateDiff function being used in the expression

The first DateTime value that will be compared in the expression. (Order is important. If you are getting negative values switch the values being used for StartDate and EndDate)

The second DateTime value being compared in the expression

Basic Syntax - Get Functions

Syntax:    GetDate(DateTime)
Example: GetDate([OrderDate])

Get function being used in the expression

The DateTime value that will be used to get X from (Date, DayOfWeek, Hour, etc.)

Basic Syntax - Is Functions

Syntax:    IsLastMonth(DateTime)
Example: IsLastMonth([OrderDate])

Is function being used in the expression

The DateTime value compared to the specified expression (LastMonth, NextMonth, LastYear, etc.)