Relative Dates are essential for Scheduled Reports. For example, scheduling a report based off of copies added in the last month will always pull data for the month prior to the date the report is run, and not a month prior to when the report was originally scheduled.
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A Date Range is the time in between two specified dates such as 01/01/17–02/15/17.
A Relative Date is a generalized time period, such as a week or a month, rather than specific dates.