Having worked at a software company whose internal date format was indeed based on days as the base unit, I suggest that this is an extremely bad idea. 0.75 may be 6pm, but what is 7pm? 0.7916666666666666 is as close as computers will generally come, but of course it's lossy; try adding increments of an hour and sooner or later you've got an irritating rounding issue.

This is solvable using arbitrary-precision decimal libraries, but relying on one of those for just dealing with general time stuff is a Bad Idea.