Timebomb

A timebomb (or time bomb) is a software mechanism that renders a computer program unusable after a set period of time. It is commonly seen in pre-release software, where it is used to discourage users from holding onto out-of-date testing versions.

Windows 9x
The timebomb is activated through. Once the timebomb is triggered, Windows will display a message on boot saying the pre-evaluation period has expired, followed by an immediate shut down. The operating system will not boot unlesss it is reinstalled with the correct BIOS date as  patches itself to enter an infinite loop. Windows 95 build 216 is the first known build to include a timebomb.

Windows NT
Once a timebomb is triggered, the  bugcheck is used to reboot the system after a period of time has elapsed. The first build to feature a modern timebomb was Windows 2000 build 1627.1, although the Japanese and Korean pre-release versions of Windows NT 3.5 include a warning message box telling the user to install a retail copy of Windows after a certain date, unlike other builds from the era.

Certain builds of Windows 8 and later versions contain a milder variant of the timebomb which does not restart the computer upon reaching the expiration date. Instead, it deactivates Windows and prevents access to personalization options in Settings, as well as disabling the ability to change the desktop wallpaper in the Control Panel unless a third-party tool is used. In addition to this, the Microsoft Confidential warning above the watermark will be positioned slightly higher than the original, indicating that the timebomb has been triggered.

Most builds of 1507, 1511, 1607, and 1703 are flight-signed (file signature checks take certificate expiry time into account). When the timebomb activates, the boot loader will be rendered non-functional due to expired certificates.

macOS
The Mac OS X Public Beta is the only known version to feature a timebomb. While the timebomb prevented the user from logging into the desktop, it did not affect aspects of the Darwin kernel or any other components that are not related to the user interface.