Note: This was an issue in early betas of iOS 10. The correct string in Chinese characters is displayed in Developer Beta 3/Public Beta 2.
The number is the current "year" in the current cycle of the Chinese Sexagenary Cycle.
Year 33 of the current cycle corresponds to
丙申 in the Stem-Branch system, which is displayed when the date is displayed in the Calendar application (both iOS and OS X/macOS).
General formula for converting years 4 AD/CE and later is
floor(western year - 3)/60), where
floor is equivalent to taking only the part in front of the decimal.
floor((2016-3)/60) = floor(2013/60) = floor(33.55) = 33
See the link for more details on the system.
In iOS, previous versions may have displayed the long string which can be seen when opening the Day view of the Calendar app.
To enable/disable this display, go to
Calendar > Alternate Calendars in the Settings app.