How soon you can apply for UK visitor visa

Happy New Year to all,

Hope everyone had nice time last year and looking forward for this year.
I am planning to apply for my father's visitor visa on August, 2018 but he is planning to travel on next year summer around April, 2019.

I wanted to know will UKVI accept visa application if you are applying 6 months before. I can't see any rule on this. please point me if there is any.

Recently I tried to create an online visa application for my father, it does allow for an application applying more than 6 months before. Not sure later if it fails or rejects as it just asks date of arrival to UK which is around 8 months later.

I will be really grateful if you clarify my queries please. Thanks in advance.

5. ECB9.5 What if applicant is not intending to travel straight away to the UK.

thank you vinny for your swift response.
As per rule, we can apply just 3 months prior to the date of travel or arrival.

Just curious to know - why does the online application allow you to enter future date beyond 3 months ?
In that case, it will be always rejected.