create a fixed daycount notation for datetime ranges

[wilsonfreitas]

I need to create a daycount to specify fixed datetime intervals.
For example, a spot rate is valid between 2 dates.
So it depends on a calendar, but daycount doesn't refer to a calendar.
What to do?

Suggestion:

as.daycount('fixed/2015-06-12:2015-07-12')

and use spotrate's calendar.

The problem is how to compute timefactor and dib that usually returns the number after /.
One possible solution is: dib and timefactor methods have an optional argument for calendar.

dib(dc, calendar=NULL)
timefactor(dc, term, calendar=NULL)

and daycount could inform it has a datetime interval with a method has_daterange, that method would be used in dib, timefactor simply passes through.
So, every time a curve use timefactor it should pass a calendar.