another way is to let ocs search for this registry key
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\InstallDate
But you must convert the unix time
for example
1357790033
01/10/2013
All the registry key can be imported by glpi
the advantage is that you can collect many different informations
for example keyboard layout, display language, os language, et cetera from registry, and you can search in glpi for all of this kind of stuff
kind regards