Seems like there is a SW issue with 64bit iPhones on dealing with time if you set the date/time to Jan. 1, 1970/00:00:00. iOS is using Unix time to track time. This means that the time on your iPhone is defined as the number of seconds passed from Unix Epoch (January 1st, 1970 at UTC) until now. This number is then converted to your local date/time based on your time zone or any other setting you might have.
Setting the time to Unix Epoch (January 1st, 1970 at UTC) seems to be a bad idea and various trusted sources confirmed this rumor. Read More