I found the answer on the CIC web;) K' w5 J5 H- K8 V, ]# p3 J
"The certificate must have been issued no more than three months before you submit your application" * T4 a4 x+ Q$ g# K: P$ }+ S1 Y( C6 ?+ y( c1 I: K1 P5 X
Ours were more than one year when we applied.