I_step=3

Hi

I want to know how I_step 3 is working exactly .

I have write a code for I_step 2 ,it is taking from date and based on the calculation ,I_step2 it is calculating to date .

I want to do an exception user should don't give From date before current date .if he will do then message will come and inform give date before today day .

Can any any please inform me how to trigger I_step 3.