How about some software release dates? Took a few days to compile this and get as accurate as I could with these!
MS-DOS release dates:
1.0 - 12th August 1981
2.0 - 8th March 1983
3.0 - 14th August 1984
3.3 - 24th July 1987 (First retail release, included some FAT16 support)
4.0 - 6th November 1989 (Full FAT16 support, handles partitions larger than 32MB)
5.0 - 11th June 1991 (Allowed partitions upto 2GB in size, replaced GWBasic with QBasic - included Gorillas.bas!)
6.0 - 30th March 1993
6.22 - 1st June 1994 (Final retail release of MS-DOS)
Windows release dates:
1.0 - November 25th 1985
2.0 - December 9th 1987
2.1 - May 25th 1988
3.0 - May 22nd 1990
3.1 - April 6th 1992
NT 3.1 - 27th July 1993 (Introduced NTFS, available for x86, MIPS & DEC Alpha architectures)
3.11 for Workgroups - November 8th 1993
NT 3.5 - 21st September 1994
NT 3.51 - 30th May 1995 (available for x86, PowerPC, MIPS & DEC Alpha architectures)
Windows 95 - 14th July 1995 (Introduced FAT32 with OSR2 on 31st August 1996)
NT 4.0 - 31st July 1996
Windows 98 -15th May 1998
IBM OS/2 release dates:
1.0 - 21st October 1987
1.1 - 31st October 1988
1.2 - 16th May 1989 (Introduced HPFS)
1.3 - 30th November 1990
2.0 - 30th March 1992
2.1 - 14th June 1993
3.0 - 11th October 1994 (x86), 5th January 1996 (PowerPC)
4.0 - 25th September 1996
Macintosh System release dates:
1.0 - 24th January 1984
2.0 - 2nd March 1985
2.1 - 17th September 1985 (includes HD20 support, adds HFS)
3.0 - 16th January 1986
4.0 - 1st January 1987 (Released with the Macintosh SE)
4.1 - 2nd March 1987 (Added support for Macintosh II)
6.0 - 12th April 1988
6.0.8 - 13th May 1991
7.0 - 13th May 1991 (yes, i know)
7.1 - 28th August 1992
7.5 - 25th September 1994
7.5.3 - 1st January 1996
7.5.5 - 27th September 1996 (Last version to support 68000 & non-32-bit clean ROM Machines)
7.6 - 7th January 1997
7.6.1 - 7th April 1997 (Last version to support 68030 based machines)
8.0 - 26th July 1997
8.1 - 19th January 1998 (Adds HFS+ support, last version to support 68k Machines)
8.5 - 17th October 1998 (PowerPC Only from here - 8.5.1 with major bugfixes released 7th December 1998)
8.6 - 10th May 1999 (Adds nanokernel)