After spending many many hours trying to make the date time format work with an ascii file, I finally tried using yyyy instead of YYYY for the year format and it worked.
The data wizard ascii file configuration page incorrectly specifies that years should be designated as YYYY when indicating a format string. The page gives the very un-useful hint that the format string should follow the specification " d=day, M=month, Y=year, h=hour, m=minute, s=second, z=millisecond". The "Y=year" specification appears to be incorrect. It should be "y=year".

I was successful in using a format string of yyyy/MM/dd hh:mm:ss:z with the following ascii file:
date time feet degC
2005/10/23 15:30:00.0 0.24 6.65
2005/10/23 16:00:00.0 0.25 5.75
2005/10/23 16:30:00.0 0.24 5.24
2005/10/23 17:00:00.0 0.25 4.83
2005/10/23 17:30:00.0 0.25 4.54
2005/10/23 18:00:00.0 0.25 4.08
2005/10/23 18:30:00.0 0.25 3.70
2005/10/23 19:00:00.0 0.26 3.53
2005/10/23 19:30:00.0 0.26 3.53
2005/10/23 20:00:00.0 0.25 3.42
2005/10/23 20:30:00.0 0.26 3.26
2005/10/23 21:00:00.0 0.26 3.24

It would save users much time if the incorrect hint on the wizard's ascii file configuration page was corrected or deleted.

As this is a bug in Kst / the documentation of Kst, I would suggest filing a bug report against Kst at bugs.kde.org.