If you look for your correct Chinese Lunar Day, then you don't need to enter city longitude for most of cases. Unless, the new moon day of the time zone is around midnight at 24:00 p.m., then you have to enter the time zone and local city longitude.
In the following link, type your name of your birthplace to find the city longitude. If the city longitude is 118e36, then use 118.36 as the east longitude. If the city longitude is 128w23, then use 128.23 as the west longitude in the Chinese Calendar Conversion screen with a negative sign.
If longitude is 121° 31' in the following link, then enter 121.31 into the Chinese Calendar Conversion screen.