This tracks Tyr's Deliverance extension in an accurate and efficient way. It looks at the actual buff being extended on aura trigger updates. WA happens to save the initial expiration time, so we can just compare new expiration time with the initial one on aura updates (which only happen when something changes with the aura).
The number in the bar is how much extension you can get, the number above the icon is overall duration.
Note that it is possible to go over 40 on the last extend - i.e. if the extension is at 38 seconds, and you cast a Holy Light adding 8 seconds, the extension will go to 46 seconds. This is not a UI bug, Tyr's can actually be extended for 46 seconds instead of 40 in this way (but probably not worth trying to play around). The bar will simply show 46/40 if you do this.