You don't need the city longitude, unless your birth hour is around 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21 and 23 o'clock.
To find your city longitude is to click the Go button, then type your birth town to find the city longitude. If the city longitude is 128w23, then use 128.23 as the west longitude in the Chinese calendar screen with a negative sign. If the city longitude is 118e36, then use 118.36 as the east longitude.
More city longitudes at Get Latitude and Longitude Site