[SalesForce] 24 hours format from date/time field

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();
}

The easiest way to do this via trigger would be to use Business hours. In this case it would be:

BusinessHours bh = [select Id from BusinessHours where IsDefault=true];
obj.SLA_deadline__c = BusinessHours.add(bh.Id, Datetime.Now(), 32400000);