There is a round off error, in this particular result, of 1 second. The round off error is an artifact of converting from hours, minutes and seconds to decimal days and back again. If you use the same dates above without the times, you get an answer of 16608 days. Which is correct since the time used was 0, or midnight on both dates. Any date and time entered on the stack that is before the current date at midnight will return a negative answer.
Positive answers are future dates and times.
Enter 19450622 on the stack and press T- . The function set added the negative days to the current date giving the originally entered date. You can also use dates with times on the dates entered on the stack with the T- function. The Delta Time Handling Functions work on a date and time and an offset in days and time to compute a new date. For example, you want to know what date was 15 days prior to February 1, 1978.