The absolute "Visible Month" setting works only with positive values so it can't be used to show months from past year. You could try to use scripting to change the displayed month. Here's one which changes the start month to July 2012 once per minute.
Rainlendar_SetDisplayDate(1, 6, 2012) -- Jan = 0, Feb = 1, ...
-- Create a timer which triggers once per minute
Rainlendar_CreateTimer(60 * 1000, ResetToJuly_OnTimer)
You can paste the code to a *.lua file and place it in Rainlendar's scripts folder and it should work after automatically.
If you want to improve the script there is a similar one in the wiki
which does things a bit better as it doesn't change the date unless necessary and works better if the display date is changed manually.