If this is truly just am image there probably isn't a way to tell RFT to click on the number 5 (for example).
What you can do is write a function that will determine the X.Y coordinates on the number you want to click on based on parameters of the image.
For example, have the function calculate the size of the image. Also you will need to come up with a constant that holds the space between each number. Finally you will have to have the function understand where numbers should be based on the month and year.
So you would call your function and tell it to give me the X.Y of Sept 3rd. It should go through all the calculations and hand back the X.Y for the 3rd in that image.
Make sense? Not quite trivial but doesn't sound like it would be too difficult to figure out quite reliably.