Calculators to find the duration between two dates are available on TimeAndDate.com, Calculator.net and ConvertIt.com, as of 2015. All three websites require the user to enter the start date and end date. The duration is displayed in years, months and days or in days only.
The duration calculator can be accessed on TimeAndDate.com by clicking Date to Date Calculator under the Calculators heading on the home page. The duration calculator can be found on Calculator.net by clicking Math Calculators on the home page, then clicking Date Calculator. ConvertIt.com's duration calculator is found by clicking Calculators on the home page, then clicking Difference in Days. ConverIt.com displays the duration in weeks and days too.