The installer's certificate has expired so you need to change the system date to some time in 2017 or perhaps even earlier depending on whether you are accessing a local recovery mode installer or the Internet Recovery Mode installer. After booting to the installer, launch the Terminal app from the Utilities menu on the menubar.
Enter the following command to set the date & time to Jan. 2, 2017 @ 11:33am (adjust the year accordingly for an earlier year if 2017 does not work):
date -u 0102113317
If there are no errors with this command, then quit the Terminal app and select the "Reinstall macOS" option.