Thanks for all responses, sorry for late reply. Your expertise @Deomsh is always appreciated, thank-you for the edit and for showing your work, wonderful, helps me understand better. Yes @Gansangriff and @RainyShadow the folder dates are by far the easiest method to quick glance determine install date. My C:\WINDOWS\SYSTEM.DAT file (hidden) also indicates created date unknown. Nothing wrong with installing Windows 98 on New Years Eve, a wonderful way to spend an evening.
Thank-you very much for your custom executable @UCyborg and script @Drugwash, they both work great. AIDA64 Extreme software wasn't trialed, download too big just for this purpose. If anyone's interested AutoHotkey was discussed on this thread before and is useful for lots of different stuff.
The Windows 98 installation timestamp correlates with the creation of the C:\WINDOWS subfolders. If anyone cares my data below.
Registry:
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion
01 a9 82 4e
Oldest files:
C:\MSDOS.--- (hidden file) April 02, 2019 9:05:20 PM
C:\SYSTEM.1ST (hidden file) April 02, 2019 9:12:04 PM
Oldest folders:
C:\WINDOWS\COMMAND|CURSORS|HELP|INF|JAVA|SYSTEM32 April 02, 2019 9:08 PM
@UCyborg executable (4kb) output:
Win9x installation timestamp: 4/2/2019 9:08:02 PM
@Drugwash AutoHotkey script (4kb) output:
Your windows 9x system was installed on 2019.04.02, 21:08:02 (manual formatting)
Tuesday 2 April 2019, 21:08:02