[SalesForce] Difference in minutes between two datetime fields

You are correct - Looking at the SFDC Object reference, Case.closedDate is a DateTime field so line 2 and line 4 will break

Looking at line 2 and line 4 - these are manipulating days, not seconds so I'd rewrite line 2 and line 4 as:

Line 2

( 5 * FLOOR( ( DATEVALUE(ClosedDate) - DATE( 1900, 1, 8) ) / 7) +

Line 4

MOD( DATEVALUE(ClosedDate) - DATE( 1900, 1, 8), 7) +

You should go to the doc and use the feedback form to point this out to SFDC

If you look at my answer to the following question here, I created a very simple time difference utility to return the time difference between two Time instances or two DateTime instances.

From there you can extend the utility with the following to get what you want:

public static String GetHourAndMinuteDisplay(DateTime startDate, DateTime endDate)
{
    return GetElapsedTime(startDate, endDate).format('hh:mm');
}

If you need to get hours or minutes separately, it's rather straight-forward.

For hours:

public static Integer GetElapsedHours(DateTime startDate, DateTime endDate)
{
    return GetElapsedTime(startDate, endDate).hour();
}

For minutes:

public static Integer GetElapsedMinutes(DateTime startDate, DateTime endDate)
{
    return GetElapsedTime(startDate, endDate).minute();
}