The DateDiff function (<DateTime>, <DateTime>, <String>) returns the difference between two DateTimes as a number and rounds, if need to, to 2 decimal digits. The returned value is always positive. Possible values ​​for the “identifier” parameter are:

• s: For difference in seconds
• min: For difference in minutes
• h: For difference in hours
• d: For difference in days
• m: For difference in months
• y: For difference in years

DateDiff(Date, Date, Identifier)

DateDiff('1970-01-01', ActualDateTime(),'s')
Output in Unix format, in seconds since 1970

DateDiff('2017-01-01T06:00:00','2017-01-02T05:00:00','h')
return value: 23 (Hours)

DateDiff('2017-01-01','2017-01-20','d')
return value: 19 (Days)

DateDiff('2017-01-01','2017-05-01','m')
return value: 4 (Months)

DateDiff('2017-01-01','2026-07-01','y')
return value: 9 (Years)