I changed the date on my mac to 1995 now date and time or start up disks wont open what can i do?

Question

i changed the date on my mac to 1995 now date and time or start up disks wont open what can i do?

Answer 1

Many system actions will misbehave, given an inaccurate date. 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 16 December 2013 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 December 16, 2013 at 9 PM your command will be the following:
sudo date 121621002013
or
sudo date 1216210013
Either one will work.
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.