Okay so this isn't done yet but I decided to post it anyway for the time being. First of all this is under two assumptions because it never says exactly what age Elspeth is.
1. I am assuming that Elspeth has just turned 14 at the beginning of Obernewtyn which is because Isobelle Carmody started to write the series when she was 14.
2. I am pretending her birthday is in Winter because she always speaks so fondly of it and Obernewtyn begins in the spring.
I have been rereading the series over the last month and a half so I could make this chart. I am only have way through Wavesong aka Stone Key.
If you can read the small writing it says each time there is a change of season and is color coded to do so. Green is Spring, Red is Summer, Orange is Fall or "The Days of Rain, and Blue is Winter.
At one point in the books, Rushton says that when he first met Elspeth that he felt ashamed for having the feelings that he had for her and in Obernewtyn it says that Rushton was approximately 19, so Elspeth just turning 14 seems like a good "ashamed age."
I will finish this shortly but from what I can see, I can only add another year onto this timeline. This timeline is basically 100% accurate.