[O] Can I ask "Which day is it?" when filtering org-agenda views?

[Trevor Murphy]

As preface, I'm aware that I may have gone too far down the rabbit 
hole.   I'm inserting many time grid lines (one per half hour) 
into my agenda views so I have a visual sense of my free / busy 
time.  But I want time grid lines that have already passed to 
disappear, freeing up screen real estate for later lines.

This was pretty easy.  I customized `org-agenda-time-grid' to give 
me a line at every half-hour interval.  Then I set 
`org-agenda-before-sorting-filter-function' to return nil for time 
grid lines where the "time-of-day" text property is in the past.  

In case that description isn't clear, I'm using (apologies for 
Gmail formatting):

(defun my:org-agenda-before-sorting-filter-function 
(org-agenda-item)
 (unless (and
              (string= (nth 1 org-agenda-time-grid) 
              (get-text-property 0 'txt org-agenda-item))
              (< (get-text-property 0 'time-of-day 
              org-agenda-item)
                 (string-to-number (format-time-string "%H%M"))))
   org-agenda-item))

As an unanticipated side effect, my function also throws out time 
grid lines when I view tomorrow (and other days) in the agenda 
view.  

Is there an additional check I can throw into my function to only 
filter if the "day" of the time grid line is today?  I tried the 
obvious (get-text-property 0 'day org-agenda-item) but it seems 
that the "day" text property isn't put into the item until /after/ 
the filter-function gets a chance to process it.

Alternatively, if my attempted solution is brain-damaged please 
let me know a better way to manage the time grid lines.

--
Trevor Murphy GnuPG Key: 0xCB06EAAF