Until now, the timer control was not available when building pages. The control itself didn't appear in the toolbox.
Unless you've been keeping a close eye on this, it's easy to miss that the timer control now appears in the toolbox and can be added to pages.
This is great news because it opens up a wide range of other possibilities. For example, I worked on a messaging app where it was necessary to carry out a periodic refresh to show updated data on the page. Without the timer the control, it was difficult to implement this type of functionality.
To illustrate, here's where the timer control appears in the page designer, beneath the 'input' group in the Insert panel.
And as we can see in the model app editor, the timer control appears as expected.
Hopefully, this post will be useful to anyone else who may be unaware that the timer control is now supported in model driven app pages.