Time handling

[Dave Parker]

I need some pointers to handling/converting time
values.  I'm reading a file from a legacy DOS
application that uses a time_t type.  This is a
typedef for a long.  I understand that this is the
number of seconds since midnight, January 1, 1970.

I would like to convert it to a double - the integer
part which is the number of days that have passed
since 12/30/1899, and the fractional part is the fraction
of the 24 hour day that has elapsed. (This is the Delphi
format, and maybe the format for other languages and
systems.  But I'll doing the conversion in C/C++).

Has this been solved before?  Where should I look?

Thanks much!