I believe a transaction needs to be confirmed/mined to count in that window. As stated on eos.io, "On the Ethereum blockchain, timing of block production is determined by proof of work so block production can occur at random times. For example, ETH sent to the EOS Token Distribution Contract in the final seconds of a distribution period may not get included for that period. You acknowledge and understand that the Ethereum blockchain may not include your transaction at the time you expect and you may not receive EOS Tokens the same day you send ETH. To mitigate this, it may be possible to use the EOS Token Distribution Interface to require the transfer of ETH to apply to the entire current period or otherwise the transaction will fail."
In other words, you could include as part of the transaction, for it to be included in a specific window, so it will be cancelled if it does not make it in time, but there is no way to ensure it will be in that window besides sending it earlier.
All the same, very good question.
Thanks for the clarification 👍🏼
BTW, it says: "To mitigate this, it may be possible to use the EOS Token Distribution Interface to require the transfer of ETH to apply to the entire current period or otherwise the transaction will fail."
Any idea how this can be done on myetherwallet? I would like to contribute in the last few mins, but to make sure it is either accepted or rejected, but definitely not transferred to the next period.