If you have delayed your EI application (applied beyond the 3 weeks grace period from the day you stopped working), you must still file you online application right away. Sometimes the online application will ask you what is the reason you have delayed your application, you must answer it accordingly. If not, you have to submit an Antedate form once you have completed your application and submit it with your ROE by mail or at your nearest Service Canada centre.
If you have a valid reason for your delay, then processing office will be able to push your claim starting date back to the week after your last day of work. Then you will have to complete the past reports in order to get paid for your retroactive payments.
If you have not been approved for your delayed application, your EI will begin from the week you applied online. In this situation, your EI benefits weeks and amount will decrease because of the delayed weeks that have zero insurable earning and hours.