Need formula: progress (% completion) between two dates

Hi, I am looking for a formula to determine the progress completion (in percent) between two dates as of a particular date. For example, if I want to know the percentage complete as of today's date given a start date of Jan 1 2010 and Jan 1 2020, is it possible to do this in excel? Also, is possible to design a formula adding specific times? (E.g. 12:00am on jan 1, 2010 to 12:00am dec 31 2019, as of now (12/17/14 at 8:20am). Thanks

adapt formula:

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and format cell as %

Thanks. Can you please give me an example so I know exactly how to enter this into excel? I am having an issue doing a calculation containing the dates (please also include times) thx!