its a carrier specific setting. at&t is one of only a handful of carriers around the world which actually support NITZ (network identity and time zone), a protocol used to set the correct time and date on mobile phones automatically.

so with an at&t sim this setting appears in settings > general > date & time.
the iphone os comes with a bunch of carrier settings for carriers all around the world. the so called carrier bundles. with these files specific carrier settings apply according to the sim card respectively carrier the iphone is used with.
so depending on your carrier certain options may or may not be available in the iphone's settings.

in the end however you can modify carrier bundle files. so you actually could activate the switch for automatic date & time. which in turn would be useless unless the carrier you're using supports this feature.

so it all comes down to the carrier supporting NITZ.