If you must calculate a date difference, now your can do this easily with 'Açana Date Calculator'. Set the first and the second date. «Açana Date Calculator» calculates  the date. Now you get the date difference in Years, Months, Days, Hours, Minutes and Seconds. You can also make an addition or substraction. Set the start date, the values for years, months, day, hours, minutes and seconds and you get the new date.

 

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«Açana Date Calculator» is also available for macOS