That is to say, Apple's servers could not verify the macOS certificate because of the incorrect system time settings. If the date and time embedded in the certificate are set before the release date or after the expiration date, or if there is no system clock has been set in the situation of a replacement of the startup disk, the Macintosh operating system will not install on your disk and the UNTRUSTED_CERT_TITLE will pop up. This is because the system wants to make sure the digital certification confirming the integrity of the program has not yet expired. When you require a copy of macOS and want to reinstall it through the Internet, Apple's servers use your system time to establish a secure connection. Some victims also reported that they received the UNTRUSTED_CERT_TITLE error message after they replaced a new startup disk on their MacBook Air or MacBook Pro. This error occurs most of the time when you are reinstalling macOS, especially macOS Sierra and High Sierra, on your Mac in Internet Recovery Mode. Why does your Mac report UNTRUSTED_CERT_TITLE? How to set the system date and time correctly? This article will explain what the error means and find practical solutions. It is not very common for Mac users to encounter UNTRUSTED_CERT_TITLE error in macOS Recovery Mode. However, when you reinstall macOS, an error UNTRUSTED_CERT_TITLE occurs to prevent you to do so. When it is done, backup the disk image to your online/local backup.MacOS can be reinstalled to fix some Mac issues such as Mac not booting up or Mac kernel panic. Select Save the home folder in a disk image and Click Delete User. Access System Preferences→Users & Groups, and Clock minus button on the bottom in the left panel. First of all, create a new admin account and login with this account. There is an easy way to backup each user account on your Mac. If you have more than one user on your Mac, and you’re only interested in backing up your own data, just go one level deeper and select your user account from within the Users folder. Each user account contains your Desktop files, documents, downloads, videos, music, and pictures. This will back up your user account, along with any other accounts on the Mac, plus any shared data. When choosing what to backup manually, most importantly look at the Users folder, located by default at Macintosh HD/Users/.

So, If you skip this step, you won’t be able to recover your data if something goes wrong.

Note: A clean install of macOS Sierra on the startup drive completely erases all data on that drive including all your files (documents, music, movies, pictures, and even apps).

Reinstall macos sierra