The date of the proposed itinerary has no bearing at all on the application... it's not a required document and is just included to show have looked at possible flights.
The only date that matters is what you list as your Intended Travel Date on the application... and as long as it is no more than 3 months ahead, they don't care.
So, for the intended travel date just put the most convenient/earliest possible date you will be physically able to travel - you will only have 30 days from that date to travel, so this is your opportunity to tell them when you want those 30 days to be (and hopefully they will honour that).
If the date has passed before the visa is issued, it will just be made valid from the date they approve it. So, if you were submitting the application today, you could put today's date (in theory) and it wouldn't matter - it will just be made valid from the date they issue it instead.
Sent from my iPhone using Tapatalk