This calculator converts number of days to number of years, months and days in the month. Of course, when you know start and end dates for an event, you can use precise version How many days are there between two dates? or Days between dates. The historical version.. But when you see or hear something like "I am 10000 days long" or "the siege was lifted after 872 days after it began" (Siege of Leningrad) you just want to get general sense of time, length of a period in general, and not actually care if it is plus one day or minus one day because of leap year, or because of starting month.
In this case, you can use calculator below. It just adds specified number of days to 1st of January 2017 (first non-leap year after 2016) and then grabs number of years, months and days in the month passed between start and end date. This allows you to make sense from number of days and understand how many years, months and days it takes.