Experimentation shows that in the context of 'today', time is taken into account.  Deadline < today is finding a note made to test your question this afternoon (in UK) - the time is 14:48. It's now 22:40. If I change the latter note's time (date stays the same) to 23:40 it's now excluded.

Doh! I didn't read the full section I quoted from the manual (I don't use dates a lot). Manual p.114, section "Agents, Dates and times":
Quote:

What's catching you out is '=' acts differently in that it ignores time unlike other comparison operators.

So time does matter but, as already discussed default time is always 00:00 unless you set it manually or through some string. I found this worked as as a test agent action:
MyDate = yesterday 09:00
..and my date attribute duly had a date of yesterday and time of 09:00. I suspect local setting for time/date may have some bearing on the time format.  Here in UK (UK OS settings in 10.4.11) I'm doing fine with 24hr hours and with a colon hour/minute delimiter.

Is time necessary on a date attribute? Could the date attribute be simplified to only include a date, which means anytime between 12.00.01 am and midnight on a particular day? Could time be a separate attribute?

Time is irrelavant to the way I work with dates--a day is fine-grained enough for me--but I'm guessing it's integral to Tinderboxes's calculations.

Matt

So the time-insensitive version of
Deadline > yesterday
is
Deadline > day(yesterday)+"/"+"month(yesterday)+"/"year(yesterday)
...(readers with US settings reverse day & month argument)

The latter syntax seems verbose for regular use (and I can't get it to work**). Perhaps those seeking time-insensitive date/time comparison would benefit from an additional date operator allowing a concise version of the above syntax, e.g.
Deadline > dateonly(yesterday)

IOW, for a today of '09/05/2008 09:07', it would return '09/05/2008'

** Presumably because I'm trying to compare a string with a date/time.

I note with interest we've parallel threads with the GTD-ers wanting both time-sensitive and time-insensitive (or rather timeless) comparisons within the GTD context.