Attackers can set send malicious NTP requests to adjust every iPhone's time settings to January 1, 1970, hence brick every iPhone connected to the same network. But this is one Easter egg you definitely do not want to hunt. Well in fact it hates it so much that it will permanently crash your iPhone's if you change time settings to the mentioned date.
Apple has said it was looking in to the bug reports. Moving an iPhone's date setting back by 46 years isn't something most people would do without some prompting.
Tech website 9to5mac posted a video demonstrating the date bug, which appears to only affect Apple devices with 64-bit processors - meaning iPhone 5S, iPad Air, iPad Mini 2 and the sixth-generation iPod Touch, or newer models of those devices. Rather than tracking time as a tradition date, iOS (and other Unix-like systems) measure it as the number of seconds from January 1, 1970. "Change the date on your iPhone to January 1, 1970, press and hold the power button to reboot your device, and prepare for a wild ride".
According to Ars Technica, this happens because 1 January, 1970 is the first day of the Unix epoch and that allowing the phone's battery to go completely dead (or disconnecting the battery) will reset the date. Perhaps it was some Apple engineer throwing in his idea of a joke thinking no one would even think about doing this.