Get time difference from specific day

I am trying to find some utility in the chrono namespace to provide to my application the same feature I had in my C# program. What I need to do is to compute the time difference between to specific dates, but I can't find anything this specific. For example following is my C# code:

var startDate = new DateTime(2000, 1, 1);
int diffDays = (DateTime.Today.Date - startDate.Date).Days;

return diffDays.ToString();

Is there any equivalent function in C++?