Hi @Sang Min Kim !
Actually, this signed date/time is always GMT 0, but it may be possible for it to show the date/time converted to your timezone selected in your user preferences.
The Administrator of your account is the one who defines which date/time will appear in the document, whether it will be the account default for everyone or whether each user will be able to see their own time zone (automatically converted).
Read this article below for more information on this topic: Time and Date Settings for Your Account
Please, let me know if I was able to help you with this question. If so, could you please select click on the "Select as Best" option? Thank you!