convert back from days to date (C++)?

I have a C++ code that turns the date back to day (considering leap years). I get a date from the user and turn it to day (my base is 1900/01/01). I thought that would make my life easier when I want to overload <, >, + and - (+&- meaning add some user defined day: add 12 days to this date or remove 301 for that date).

Now I am trying to wrote a method to turn back the result day to date; and here is the place I got stuck; How should I go about it (not using pre-defined libraries such as time.h)? is it possible at least?
Just do the opposite of what you did for the forward conversion. It should be fairly straightforward. What's the problem?
the problem is you so truly put it is I do change that number; the number that I get from the date the user inputs. I add or remove some days from it (overloading -/+ operators);
at the end I don't have the idea any more from which date this number was originated from.
it is a number and a base which is 1900/01/01;
how many 365 should I take from it how many 366?
Do I see the problem wrong?
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