date validation

hi,

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.