Why do I have to add 1+ to calculate elapsed time?

In the 6th video of Excel with Formulas chapter in Excel for Data Analysis course, topic named “How to perform calculations on time and completion of year percentage using excel spread sheet formulas”, it is said that we have to apply the formula =if(B2<A2,1+B2-A2,B2-A2) to calculate elapsed time. Sir said that, 1+ is used for negative values. Can you please elaborate it with an example as what is the importance of this 1+?