Belgium Dependent Visa

Hi,

I came to Belgium with work permit on 1st week of May and my visa will expire on end of dec 2018. I have applied dependent visa along with me and their visa will expire on 08th Oct.
I am planning to get my family to belgium for 1 month (in September). Is there any policy/rule that my dependents have to travel to Belgium within 3 months of their visa issued date?

Thanks
Venkata

Did you read somewhere on such a rule ?

In my knowledge, they can enter anytime during validity period of their visa.