farmtalk is correct. There is a base temperature used for each of the two measurements which is supposed to be your inside house temperature. The cumulative difference of the actual average air temperature from that base temperature is either the heating or cooling degree days. Those two measurements are then directly proportional to the amount of energy used for heating or cooling.
If your propane supplier is using an incorrect base temperature for your residence, e.g. too low, then they would underestimate your heating degree days, your energy usage, and allow your propane to run out. Also if your propane supplier is using daily temperatures that are warmer than those you are reading at your place, they would also underestimate your propane consumption.
You have to figure out your average house temperature which can be tough to do if you use a set back thermostat and then apply that as the base temperature in Weatherlink. Then you have to relate the heating degree days that you measure to your propane consumption rate. Once you have that relationship, you can "project" how soon your tank of propane will run out if you forecast the heating degree days that might occur in the future.
I believe the Fuel Demand calculator in weatherlink does most of this (it doesn't do fuel consumption projections), but I've never used it properly. I tried setting it up for natural gas which I think should also work, but gave up. The problem was estimating the "K factor" for natural gas. Your fuel supplier should be using a program like the Fuel Demand calculator.