You may make an offer on C2FO at any time. However, offers are submitted to your customer only once per day, at the daily market clear time. You have until then to make any changes to your offer.
Market clear times are based on the location of your customer and are as follows: Americas: 12:00 PM ET
Europe / Middle East: 5:00 PM GMT
Asia-Pacific: 12:00 PM HKT
Entering an offer before that time will simply allow you to see whether your customer would accept your offer. However, that offer is not submitted and finalized until the market clear time. A countdown clock lets you know the time remaining until the next market clear time.