IHS - charges Tier 4 Dependent new born baby - Urgent

Hi
Please advise as My Wife is Main applicant Tier 4 and applied before 6 april and granted visa till 2018 including 2 x dependent.
Now I am applying for my new born baby as Tier 4 Dependent to get IHS ref no but when I apply here :
www.immigration-health-surcharge.service.gov.uk
and come up with course start date as 13 April 2015 system doesn't let me go ahead as it says enter valid date.
Please advise what to do ?

Thank you in advance