You don't have to log it on that date, just with that date. There's a grace period of usually a week or two in which you can back-log a cache and still receive the souvenir.
@ianian: Did you log it with another date, then edit the log and change it to the 18th? If so, the system won't award a souvenir. Either way, you can email contact@geocaching.com and ask them to manually award the souvenir.
I think it is the problem of the time zone difference. I found the cache on 15:00 18th. I log the found around 16:00 18th. But it was the China time. I thought that made the difference. I will contact the contact@geocaching.com. Thanks.