Q: My son changed the date to 1970 on his macbook Air and now nothing work and can't change it in system preferences. Is there any other way to change it back without having to restore everything?

See if you can change the date in Safe Mode.

Startup the Mac in Safe Mode

A Safe Mode boot takes much longer than a normal boot so be patient.

Once you are in Safe Mode open System Preferences > Date & Time > Date & Time then select:  Set date and time automatically

Then click Restart from the Apple () drop down menu.

Many system actions will misbehave, given an inaccurate date. Resetting the NVRAM and / or SMC will not fix it, and if the system has a preposterous date you may not be able to open System Preferences to correct it. This dilemma may or may not be addressed in a future OS X update.

If you cannot open System Preferences you can reset your system date and time using Terminal.

Please read everything that follows before proceeding. Write them down if necessary because the effects of setting a preposterous system date are unpredictable.

Quit System Preferences, force quit if necessary: OS X: How to quit an unresponsive application using Force Quit

Open Terminal - it is in your Utilities folder and looks like this

You can find Terminal by using the Finder's Go menu and choosing Utilities, then double-click the Terminal icon.

At the prompt, you will be typing a command resembling the following:

sudo date mmddhhnnyyyy

... but you must substitute the actual date and time in the command that corresponds to the date format used for your location. In other words if your custom is to use a date like 10 May 2014 then reverse the mm and dd values.

Substitute numerals in the above command according to the following:

mm = month

dd = date

hh = hour (24 hour format)

nn = minutes past the hour

yyyy = year

Each of those values must be two digits except for the year which can be two or four digits. As an example if the current date and time is May 10, 2014 at 9 PM your command will be the following:

sudo date 051021002014

or

sudo date 0510210014

Either one will work. If you enter nonsensical values the system will attempt to parse the entry correctly.

To recover from your inability to use System Preferences to set the date and time, the value entered does not have to be precise, it just has to be approximate.

Enter that command using the actual time and date for your location and in the appropriate format, then press the Return key. Terminal will ask for your Admin Password - the same one you use when you log in to your Mac. Type it and then press the Return key. What you type will not appear, not even with •••• characters.

To determine if your change was effective just type the date command and press the Return key. Terminal will reply with the date and time. If it appears correct and in the correct format, you should be able to open System Preferences > Date & Time once again. Selecting the "Set time and date automatically" and choosing an appropriate time server will adjust your Mac's system clock to the precise time.