All .exe files have same modification date

This may be the case that is some kind malware, but it is very unlikely.
If you have any program, scan for viruses and malware and as well clean the registry and all remove all temp files as much as possible.
then restart computer and then check what has happened.
PS If something has already happened, nothing can get worse. There is a possibility that the problem is caused by junk files and obsolete registry entries.
You can only do this, of course, when these programs work, what You need for cleaning.
Such a things may sometimes happen, if computer isn't turned off for long time and is used only sleep or hibernation modes.

 

Do not install any third parti antiviruses if possible. Let Windows default antivirus, ie Windows Defender Security Center runs, it's enough. If You need, there is many scanners, what can be used if needed, but usually not needed.
Can You post some example. What exactly has changed..I mean some .exe file. Upload for example to upload.ee, i'll look on it.

 

It may be a symptom of a malware. Try to install another antivirus (Avast Free) and start a throrough analysis by including rootkits and safe mode if necessary.