Hello Mua,
How did you change the default_time_zone flag, exactly. Did you use the command line with gcloud sql instances patch [INSTANCE_NAME] --database-flags [FLAG1=VALUE1,FLAG2=VALUE2]? Did you use Cloud Console? Worth noting: leading zeros required.
You mention echoing the date time. What is the output of SHOW VARIABLES; ? What is the value of system_time_zone? Are values of the two flags identical?
This discussion group is oriented more towards general opinions, trends, and issues of general nature touching the app engine. For Cloud SQL setup, you may be better served in dedicated forums such as stackoverflow, where experienced programmers are within reach and ready to help.