Why does the install date and size of a program change roughly two days after I install it, even though no changes were made to the program in the meantime?

A customer observed that when they install a particular company’s product, and then go to the Programs and Features control panel, the program shows up with the correct installation date. But wait a few days, and then installation date jumps forward roughly two days, even though no updates for the app were installed in the meantime. (Also, the file size changes.) What’s going on?

What’s going on is that the system is trying to improve upon the incomplete information it was given.

If a program does not provide size information, then the Programs and Features control panel starts guessing by doing textual matching between the name of the program in the Start menu and the name of the program in the Programs and Features folder.

And if a program does not provide installed-on information, then the Programs and Features control panel assumes that the file was installed (or last modified) the last time its uninstall information was modified. There is no creation time for registry keys; last-modified time is all you get.

The final piece of the puzzle is that in Windows 10, the Storage Service looks for programs that registered with the Programs and Features control panel but didn’t provide any size information, and it works behind the scenes trying to do a better job of figuring out which files on the system are part of that program, and when it comes to a conclusion, it updates the registration with the updated size information.

Okay, now it’s time to connect the dots.

You install a program that provides incomplete information in its Programs and Features registration.

Some time later, the Storage Service figure out a good estimate for the size of the program. The Storage Service then updates the registry entry with the improved size stimate.

Updating this information causes the Size column to change.

And if the program did not specify an Installed On date, it also has as a side effect of changing the last-modified time of the registry key, which causes the Installed On column to change.

To avoid this problem, just make sure your program fills out both the InstallDate and the EstimatedSize fields when it registers with the Programs and Features control panel.

Of course, if it’s not your program, then you either have to lobby the program authors to do it, or you can just dive in and set the values yourself.