Timer Dialogs are to be used when you have disabled the Game UI for a custom UI. This is more complex than Timers.
Define Timer Dialog
Timer Dialogs are used in a Dialog when the Timer Windows are not visible. You can do this using 2 different forms. You can either use a Real form (##.###) or an Integer form (##). Real forms for to the third decimal and can be made with some small changes.
You can activate a Timer Dialog in the middle of the game by having it "Initially Off" (Right Click the Trigger and Uncheck). Then at the end of this trigger, have the Action: "Turn (Trigger) On" which will be the following trigger.
Creating the Dialog
First you want to actually Start a Timer, create a Dialog, and then make sure you have "Show Dialog" trigger on. Then you want to "Add Dialog Item(Label)" to the Dialog, and that will be our Explaining Label. For this label, you can use something similar to the Timer Window, eg. "Executing in:". Make another Label and this will be our numbers. Make a Global Variable and set it's type to Dialog Item, Set this variable to out Number Label.
Create and Destroy (Integer)
If you chose to make an Integer Timer Dialog, then you have to go through 2 Conversions. In the text part of the label, choose "Convert Integer to Text" then "Convert Real to Integer" and select "Timer - Remaining Time of Timer" and select your timer.(Remember to set a variable for your timer!)
Now you want to create a brand new trigger that will be a Periodic Event for every 1 Second. You will "Destroy Dialog Item" you Number Label and then Copy and Paste the same action you had to create it initially. This will make sure that it is in the same place, at a different number. Then you will set the variable again, so this periodic event can rinse, and repeat.
Create and Destroy (Real)
This is a Similar conversion to the Integer, except that it will only go through one. In the text part of the label, choose "Convert Real to Text" then select "Timer - Remaining Time of Timer" and select your timer.
That is the only part that is different in a Timer Dialog with Real. It may be off-track and not be as efficient as a Timer Dialog with Integers though.