Here is a list of timebombs for betas in this repository.

Timebomb is a point when beta system stops functioning, so it cannot be leaked.
Set the date a month or two before it to ensure it works.

Most of the old beta images are not bootable, so you need to manually prepare the disk space and crap.
I strongly recommend using emulators for anything below Windows Whistler.

Date formatting: YYYY-MM-DD
Look up additional info in the parentheses.

Windows Chicago M4-58s: No timebomb
Windows Nashville Beta-999: No timebomb
Windows Memphis Alpha-1351: 1997-04-01
Windows Millenium DR-2332.2: 1999-12-15 (RBDC9-VTRC8-D7972-J97JY-PRVMG)
Windows Millenium B1-2394: 2000-03-15 (RBDC9-VTRC8-D7972-J97JY-PRVMG)
Windows Neptune M2-5511: 2001-02-26 (W7XTC-2YWFB-K6BPT-GMHMV-B6FDY)
Windows Janus PB-1515.1: No timebomb (Incredibly buggy ACPI! Turn it off in the BIOS)
Windows Whistler PB-2202: 2001-04-22 (Is bootable, but causes BSOD if installed directly from CD)
Windows Whistler B2-2419: 2001-07-13 (RBDC9-VTRC8-D7972-J97JY-PRVMG)
Windows Longhorn M7-4074: 2004-10-23 (TCP8W-T8PQJ-WWRRH-QH76C-99FBW)
Windows Longhorn B2-5308.50: 2007-03-07 (WGDJW-B8DYC-WVKX4-6MKF4-B8PK8)
Windows Vienna PM1-6469: 2008-04-07
Windows Jupiter M1-7850: 2011-02-05 (Use a Windows 7 Retail serial)
Windows Threshold PTP-9780: 2015-07-15 (Use a Windows 8.1 Retail serial)

Have fun exploring the betas, if you want, you can check out my channel.
Thank you for downloading from Enderman's website ^_^