Date validation


i have a date which is entered by
the user, and i calculate the date entered + 1year, say for example
start_on = 27/08/2007 and the end_on is calculated automatically as
26/08/2007 i want to check this in the model

check if the the end_on = 1year + start_on how to do this? i am using
date validation plugin. how to do this? can anyone help me.