what you're actually trying to do is:
if(
isequal([date imported],<=15d,8),0-15 days,if(isequal([date imported], etc. etc. etc.
------------------------------
Unfortunately, we cannot do that with date related fields because the database actually contains a long string of numbers for these fields, called "the raw data" and MC translates these numbers into something meaningful for us when we ask to view it. To see an example of what I'm talking about, select a photo and have a look at its 'format' properties in the tag window. You should see something like:
<Date>38731.846655092595</Date> which MC translates as 14/01/2006 20:19
For [date imported], we would need to know what the raw data was, then build around that, which is not really very practical, so, we should look to see how else we can get at what you're after.
Many moons ago now, I made 4 smartlists, popped them into a playlist group and used that group as a pane in a "recently imported" viewscheme,
and this approach still works today. Naturally, you could include as many playlists as you wish, though you should number them to keep them in a logical order when displayed in a pane, and not too many as it will make the view scheme too slow to use.
I'm currently using a calendar type viewscheme for all media imported:
Year, Month and Day are all expression based panes:
Year: FormatDate([Date Imported,0], yyyy)
Month: FormatDate([Date Imported,0], MMMM)&DataType=[Month]
Day: FormatDate([Date Imported,0], dd)
You could also use the formatdate expression to get "time ago" values:
formatdate([date imported,0],elapsed) ago
Though I don't like using those in panes as MC cannot sort the values correctly, and you get intermingled days, years and minutes and hours.
---------------------------
Hopefully, that's made things a little clearer for you.
If you really want "0-15 days ago" style grouping, you'll need to use the playlist group method. I'm undecided at the moment which I prefer best between that, or the calender view method. I'm leaning towards the calendar view, but that could just be because it's "newer". Time will tell.
-marko.