This is a hard work. But i tell to you about this metodology.
You going to create an Interface also declare property as Timer. Now you implement every page code behind class this interface. Same as signature below (IMasterTimer is posit your interface)
Public Default_aspx: System.Web.UI.Page, IMasterTimer
The interface create a property in page file as Timer. You open master page and find load function body. Convert to
Page
object to your interface. You have to give interface property from master page timer object. Same as below
var imastertimer = (IMasterTimer)Page;
imastertime.Timer = Timer1;
After you go to the page code behind file. Now firing timer on page load or another method body.