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Question:
How can I use the AddDate() function in an Arithmetic Expression to manipulate DateTime variables I have in a workflow?

Answer:
In a workflow, sometimes it's necessary to have to manipulate a DateTime variable for purposes of setting up a Time Delay step for example. 
In order to manipulate a DateTime variable, so that we can add hours to DateTime for example, we'd use the AddDate() function which will look something like the following, 

 

NOTE: This will only work for DateTime fields.
AddDate(DateInterval.Hour, Number, DW_FIELD)

DateInterval.Hour = This indicates what we're looking to modify from the DateTime. In this case, it's the Hours.
Number = This indicates the number we want to add to the interval.
DW_FIELD = This is the DateTime field that we want to manipulate. 

When this is entered into the Arithmetic Expressions section, it will add to the hours of the DateTime variable depending on the number given. 

In addition to hours, the following intervals are available to use, 
Day
DayOfYear
Hour
Minute
Month
Quarter
Second
Weekday
WeekOfYear
Year

Additional information on this function can be found at this link.
Microsoft DateAdd Method Documentation