While that can be the case in some scenarios, it does not always hold true. There is another major reason why this error message can occur and that is the digital signature that comes with the installer. We will be discussing the causes in detail down below. Users have tried downloading the installer again but in most cases, it does not work out either. The solution to this issue is pretty simple and can be easily implemented. However, before we get into that, let us go through the causes. With the causes of the error message now out of the way, we can get into the different methods that you can implement to install OS X El Capitan successfully. Follow through.
Method 1: Change Date and Time Through Terminal
The first thing that you should do when you face the error message is to change your date and time. As it turns out, you will have to change your date and time to the time when you actually downloaded the installer. That way, the certificate will still be valid and you will be able to complete the installation. Now, there are two ways that you can do this. First, if you are able to boot into your current version of macOS, you can just go to the System Preferences and do it from there. In case you can’t, don’t worry as you can still achieve through the macOS Recovery menu. We will go through both of them, so just follow through. It is important to note that when you change your date, please make sure that your Mac device does not have an active internet connection. This is because if you are connected to the internet, the date and time will just revert and you won’t be able to achieve what you want to. Once you have made sure that you do not have an active connection, follow the steps given down below: In case you are not able to boot into macOS, you can still change the date through a Terminal window from the macOS Recovery menu. To do this, just follow the steps down below:
Method 2: Force Install through the Terminal
Another way that you can get rid of the said error message is to just force install the operating system. This can be done by using the Terminal to use the installer and perform a force installation. You should only do this if you are sure that the installer is not corrupted. The method only works if you have a bootable USB stick. Follow the instructions down below to do this:
Method 3: Upgrade through Safe Mode
Finally, if the above solutions do not work out for you, what you can do is download and install the operating system in macOS Safe Mode. This is only for the users that are trying to upgrade from their current version to El Capitan. This means, if you do not have macOS installed already, this will not work for you. To do this, follow the instructions down below:
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