I take it you are not talking about CAE accreditation?
For the usual BGS CAE annual test, to have continuity it would be helpful to have your goats tested on or before the date they were tested last, so you need to do them before 17/3/17. Also best to do the adults BEFORE the kids are due, in the unlikely event of the dam not surviving the birth the kids are still covered on their dam's certificate until they turn 1yo. Once you have two years of consecutive clear tests you can apply for the BGS monitored scheme, which will reduce your testing to every two years, rather than every year - a significant cost saving once your herd builds up. Info on the BGS website.
However if you are not interested in BGS monitored scheme, then you test them whenever - for shows and matings etc you need to have a valid CAE certificate from within the last 12 months.