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No, avast is not the only software that shows this behavior.
I’ve seen it with multiple others as well.

I would have to decompile things and look at the source code to be sure (which I am not gonna do ofcourse), but I suspect it has to do with the way a application is requesting the time(zone) from Windows.

Just a simple example on what could be going on:

  • The bios/UEFI stores the time/date
  • Windows gets the time/date from there
  • Windows stores it a default time/date somewhere (or perhaps even in multiple places) *1
  • Windows stores somewhere else what timezone you have set/use and if there is DST or not. *2
    So if a application looks at *1 for the date/time it can be that it will get another date/time than when it looks at *2

As I said, I’m not sure but it seems to me something like this is going on.

About DST, sure it is old and thing have changed (a lot).
Strange thing is that there is no-one who really has calculated if it still is helping to safe energy these days.
I remember having two different prices for energy.
One was for the use between (iirc) 07:00 and 19:00
And the other was for the use between 19:00 and 07:00.
Well, the “night charge” is now only between 23:00 and 05:00
There are also a lot more electrical devices in a household nowadays and as many people are at work during the day and home at the evening/night, that also will influence the effectiveness of DST a lot.
Newer versions of old devices are using less power etc.
I would say it really is time for some scientists to have a good look at it and do the math for the different countries.