Quite serendipitously, I came across Mr. Pearson's website yesterday, while looking up on this topic.

Several work-arounds are explained there. I have developed some on my own. However, the fact that Microsoft Excel is very poor at handling time intervals remains. A very serious feature deficiency in such a widely used software.

In explaining Time Intervals, Mr. Pearson says - "However, since Excel cannot handle negative times, you must use an =IF statement ... ". Why the hell not? A negative number for money, distance, weight, ... these are not exotic concepts in calculations. So is time. Why should 10 seconds be represented as 00:00:10 and -10 seconds result in apoplexy? (Just to make matters perfectly clear, my complaint is not with Pearson, but with Microsoft)