I'd think these would make good milestones on the timeline:
The epoch, Jan 1 1970
And from the nice time line posted at UNIX History
AT&T Labs Research UNIX Time-Sharing System First Edition (V1) November 3, 1971 <---- added to FB timeline by Neo
AT&T Labs Research UNIX Time-Sharing System Second Edition (V2) June 12, 1972 <---- added to FB timeline by Neo
1BSD March 9, 1978 <---- already on the FB timeline
XENIX OS August 25, 1980 <---- added to FB timeline by Neo
SunOS 1.0 February 1982 <---- added to FB timeline by Neo
UNIX System V R1 January 1983 <---- added to FB timeline by Neo
HP-UX 1.0 (S500) H1 ??? 1983 <---- added to FB timeline by Neo
AIX/RT 2 ??? 1986 AIX Version 1, introduced in 1986 <---- added to FB timeline by Neo
SunOS 4.1.1 (Solaris 1) November 1990 <---- added to FB timeline by Neo
Linux 0.01 August 1, 1991 <---- first published Linux kernel published in October of 1991
386 BSD 0.0 February 1992 <---- added to FB timeline by Neo
FreeBSD 1.0 December 1993 <---- added to FB timeline by Neo
OpenBSD October 1995 <---- added to FB timeline by Neo
Darwin 1.0 April 5, 2000 <---- added to FB timeline by Neo
OpenSolaris (build 21) July 26, 2005 <---- OpenSolaris first released May 5, 2008 added to FB timeline by Neo
iPhone OS 1.0 June 29, 2007 <---- added to FB timeline by Neo
Android 1.0 September 23, 2008 <---- added to FB timeline by Neo