Cannot install Mojave or High Sierra on a iMac

Set the date to about one year ago. I just tried Nov 07 2018, for example. It did not work with year 2008 or with today's date. This is not the first time this issue has occurred over the past years. Remember to disable wifi or any network connection or the date will be readjusted to today's date automatically, breaking the install again.

cat burlow wrote:

Is it a hardware problem ?

It's possible, but there can be a variety of reasons your iMac became slow. Did you try simply using macOS Recovery?

"Damaged" sometimes means the system time and date are so inaccurate the installer's certificate is considered invalid. Boot the system in the same manner as before and set the system real time clock using Terminal. To determine and / or set it use the date command.

You set the date to March 3, a date in the past. Try setting it to today's date.

I cannot use macOS recovery anymore, the disk is formatted.

Assuming your Mac's hardware is otherwise operational and that its firmware has been kept up to date, macOS Recovery will work even if you were to physically remove its hard disk and replace it with a completely empty one. The Mac's firmware will determine if the startup disk or its Time Machine backup contains a Recovery Partition. If it is damaged or missing it will download the installer from Apple's servers. A viable Internet connection would obviously be required.

Reset the SMC and then try again.

The possibility of a hardware problem appears increasingly likely: Contact Support

This is known issue. We have the same issue even when downloading the Mojave installer directly from Apple. I talked to Apple Enterprise Support. Their senior engineers are "working on a solution".

Glad I could help!