The date setting in the registry for the installation date of the OS for NT is located in "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\InstallDate". The values displayed are the following example: 0x39c24d62(969035106).
InstallDate REG_DWORD Time (in seconds) since 12:00 A.M. on January 1, 1970
Stores the date and time that the current build of Windows NT was installed. For example, Monday, January 01, 1996, 12:00:00 AM would be 0x30E76A50 (820,472,400 seconds).