Date Calculator is a genealogy tool for date calculations. Although the application is intended for genealogists, other users can benefit from it as well. What the application does is let you calculate the time between 2 or more dates quite precisely. There are other features that also deal with time calculation.
Date Calculator is free for registered users of GEDitCOM II. For those who are not users of that app, the application will work with its full functionality indefinitely, but it will ask you to register each time you run it.
Now, down to business, Date Calculator is a single-window app in which you add a date and can have it converted to time in days only, years, months and days, or years and days. You can also calculate the time that has passed between two dates or more. The app lets you input months by using the buttons on the main window, and you can input days and years with the number keys of your keyboard. On the website, there is a complete list of all the calculations that this tool can make.
Although users of genealogy can benefit more from it, other users will probably like this tool. If not, there's always Wolfram Alpha for date calculations.