How to install new OS if I don't know my user password?

Everything went perfectly until the very last step, then unfortunately I got an error message saying, "This copy of the install OS X El Capitan application can't be verified. It may have been corrupted or tampered with during downloading." Any suggestions for what to do next?

To be specific: I downloaded, with no problems, the "InstallMacOSX.dmg" file directly from the Apple site, then double-clicked it and followed the instructions to create the "Install OS X El Capitan" file in the Applications folder, then took a 16gb USB drive that was formatted correctly and followed the instructions to make it a bootable drive with Terminal, and everything worked perfectly, and the USB drive became a bootable drive. Then I plugged it in to the iMac and held down the option key as I started it up, and the computer was able to find the bootable drive as hoped, then I selected it and everything proceeded perfectly, starting to install, agreeing to the license agreement, etc., until the very last step, when, just as the installation began, I got the error message above. What should do next?

Further update:

I started completely from scratch, and did everything correctly, but still encountered the same mystifying problem at the very last step. To be specific:

• Reformatted the 16gb USB drive in "Mac OS Extended.(Journaled)" format
• Downloaded a fresh brand-new El Capitan "InstallMacOSX.dmg" file directly from the Apple site, successfully
• Double-clicked it, then double-clicked the "pkg" file inside, and successfully installed the "Install OS X El Capitan" software inside the Applications folder
• Plugged in the USB drive
• Ran Terminal, followed the instructions exactly as on Create a bootable installer for macOS - Apple Support, everything worked perfectly, the USB drive was formatted to became a boot drive called "Install OS X El Capitan"
• Plugged the USB boot drive into the iMac onto which I want to install the OS, turned it on, held down the Option key, the iMac successfully found it and presented me with the drive-selection window.
• I chose the "Install OS X El Capitan" drive, clicked the arrow; got an Apple logo, then several minutes of progress bars as the drive loaded (as expected)
• Once the "OS X Utilities" window appeared, I went to the Wifi icon at the top and successfully connected to the local wifi network
• In the OS X Utilities window, I choose "Install OS X"
• Then the "OS X El Capitan: To set up the installation of OS X 10.11, click Continue" window appears as expected, and I clicked "Continue"
• Then I agreed to the Software License Agreement in the following window
• Then I selected the iMac internal disk, called "Macintosh HD" (the only option available) in the disk-selection window, and clicked "Install" to start the installation process.
• So far, everything has been done correctly and everything has gone smoothly and the process should then complete. The boot drive is correct and nothing is wrong with it, the booting process is correct and all the steps have gone exactly as they are supposed to go. And then...
• As the installation begins, I get an error message saying "This copy of the install OS X El Capitan application can't be verified. It may have been corrupted or tampered with during downloading."

Drat! The error message can't be true -- it hasn't been corrupted or tampered with.

Why do I get the message and what can I do to fix or workaround the problem?

But I did erase the iMac's "Macintosh HD" volume and then its entire physical drive, immediately prior to creating the bootable USB stick. I also, in between those two steps, changed the date of the iMac to 2017, as previously recommended. But after erasing the entire physical drive, there's no longer any way to boot into Recovery mode -- so how can I possibly change the date on the iMac now? Or is it no longer possible? There is no software, or any files at all, on the iMac now.

Prior to all this reformatting, the iMac was functioning perfectly, with no problems, and I ever ran Disk Utility on it before all these steps, and it reported zero problems. So I really don't think the drive is failing.

Yeah, sorry, I've already been through that support page, to no avail. Creating a bootable drive is the last-ditch solution after gong through many other steps and attempts, even including contacting Apple Support and doing a "Chat" with a support person (who had no new or helpful suggestions other than "take it to a repair shop and ask them"). The iMac's drive is completely wiped now anyway, so no need for fiddling with lost passwords.

Thank you for the thoughtful and very knowledgable answer! I'd give you more "Helpful" stars, but the Discussions site only allows two "Helpfuls" per question.

Tomorrow (busy for the rest of the evening here) I will try to change the date using the Terminal in the Installer (hope I can find it), first to 2022, then '21, then '20, and see if that fixes the issue. Yes, the error message happens immediately after selecting the "Install macOS" option.

You're infinitely more knowledgable about Apple software than even the Apple Support "Chat" person, who was polite but who only knew how to paraphrase the text in the Support page, with no additional savoir faire.

Thanks for the detailed reply. When I try to "Erase" the physical drive in Recovery Mode (exactly as you describe), I get an error saying "Can't Unmount Disk." Any way around this error?

Nope.

OK, I will try those things ! Thx. May take a while, though. Will eventually report back with the results.