I have a Shelly Plus1PM and I have found that, when using the device web interface on a Firefox browser, to set a ‘basic schedule’, the times do not set correctly.
When saving the schedule, the time saved seems to be the previous one to the one entered. So if I just set the time using numbers or up and down arrows and save it in the obvious normal way, the time does not change from the previous setting. And if I change the settings several times before saving, I don’t get the last chosen time values.
So, on Firefox on Windows 10, to overcome this problem, I use the up and down arrows to set the time to what I want. Then I click away from the time entry. Then I click back and use the arrows again to set the time to any other random value. Then I save the schedule, and voila, I have the time I want.
And on Firefox on an Android mobile phone to overcome the problem as the page is different, I use the clock to move the hands to the schedule I want. Then I click OK. Then I open the clock again and move the hands to anything random. Then I select OK and save the schedule, and I get the time I want.
This doesn’t occur when using Microsoft Edge. I have not tried other browsers or system hardware/software combinations.