The code is not to complex for the compiler.
But it seems, there are missed some brackets

Also I would suggest some slightly changes:

bit IsLeapYear( unsigned int y )
{
         if( ((y & 3) == 0) && (((y % 100) != 0) || ((y % 400) == 0)) )
              return( 1 );
         else
              return( 0 );
}

And I faced never problems on testing my 8051 routines with an old Borland-C 3.0 DOS version.


Peter