I don't have a WM100 so this is just a guess.
I have a Hunter-Douglas Platinum Gateway that, like the WM100, has no internet based clock and gets it's time from connecting to the phone app.
I found that if I don't connect to the gateway with the app for a few days, the times for the shades to go up or down will get a little later each day until I reconnect and the clock is updated. To solve that I just do a quick connection each day to keep it accurate.