Hmm... I think I see what you're getting at.

I like the idea of proposing a start date that is down the road, rather than propose, accept, start all at once.

How about this; the Propose War page would allow you to select the KD you want to proposal, as well as a date. This date would be limited to a range from +1 tick from the Current, to +169 ticks (1 week from 'next tick') from the Current. Having date range much longer leaves too much room for KDs to move out of War range.

The offer still needs to be accepted the same tick that it is offered, else it will expire. If an offer expires, a new one may be issued the following tick. If the offer is rejected, you may not proposal that KD for another 72h (they could still proposal you).

Once a Proposal is Accepted, neither KD in the agreement can offer Proposals, or be Proposaled.

Hostiles outside of the agreement can still escalate, and override a War agreed through Proposal, cancelling that agreement. (This should be the ONLY way to nullify an agreement made through the Proposal system)