To break it down a bit further, the code that was provided is specifically trying to get the last day of a month, which I'll call Month X since it will vary. The code is doing these things, in this order:

1. Get the month after Month X
2. If Month X is 12 (aka December) get Month 1 instead (aka January)
3. Get the Date that is day 1 of the Month from step 2
4. Get the Date that is one day before the Date from step 3

All this to get the last day of the month from Month X. The reason they did it this way is so they didn't have to say "Is this February? Then get day 28. Is this January/March/etc? then get day 31." and so on.

The code that the other user provided will instead get the last day of Month X without having to do all those steps. It's doing something in the background to get the same data, but the coder doesn't have to worry about exactly how because they can trust it will work as expected.

It ultimately boils down to the user carving out a round piece of wood, fitting it on an axle and bolting it on, then to find someone already has cheap wheels for sale that are more stable than what they just made.