Relative dates support.

README.org

@@ -191,7 +191,7 @@ These selectors take one argument alone, or multiple arguments in a list.
 +  =:category= :: Group items that match any of the given categories.  Argument may be a string or list of strings.
 +  =:children= :: Select any item that has child entries.  Argument may be ~t~ to match if it has any children, ~nil~ to match if it has no children, ~todo~ to match if it has children with any to-do keywords, or a string to match if it has children with certain to-do keywords.  You might use this to select items that are project top-level headings.  Be aware that this may be very slow in non-daily/weekly agenda views because of its recursive nature.
 +  =:date= :: Group items that have a date associated. Argument can be =t= to match items with any date, =nil= to match items without a date, or =today= to match items with today’s date. The =ts-date= text-property is matched against.
-+  =:deadline= :: Group items that have a deadline. Argument can be ~t~ (to match items with any deadline), ~nil~ (to match items that have no deadline), ~past~ (to match items with a deadline in the past), ~today~ (to match items whose deadline is today), or ~future~ (to match items with a deadline in the future). Argument may also be given like ~before DATE~ or ~after DATE~ where DATE is a date string that ~org-time-string-to-absolute~ can process.
++  =:deadline= :: Group items that have a deadline. Argument can be ~t~ (to match items with any deadline), ~nil~ (to match items that have no deadline), ~past~ (to match items with a deadline in the past), ~today~ (to match items whose deadline is today), or ~future~ (to match items with a deadline in the future). Argument may also be given like ~before DATE~ or ~after DATE~ where DATE is a date string that ~org-read-date~ can process, inculded relative dates, like ~mon~ (closest monday in future), ~-1~ (yesterday), ~+1m~ for the next month, ~+2w~ two weeks and so on.
 +  =:effort<= :: Group items that are less than (or equal to) the given effort. Argument is a time-duration string, like ~5~ or ~0:05~ for 5 minutes.
 +  =:effort>= :: Group items that are higher than (or equal to) the given effort. Argument is a time-duration string, like ~5~ or ~0:05~ for 5 minutes.
 +  ~:file-path~ :: Group items whose buffers' filename paths match any of the given regular expressions.
@@ -206,7 +206,7 @@ These selectors take one argument alone, or multiple arguments in a list.
 +  =:priority<== :: Group items that are lower than or equal to the given priority, e.g. ~B~.
 +  =:property= :: Group items with a property, optionally matching a value.  Argument may be a property name string, or a list of property name string and either value string or predicate with which to test the value.
 +  =:regexp= :: Group items that match any of the given regular expressions.
-+  =:scheduled= :: Group items that are scheduled.  Argument can be ~t~ (to match items scheduled for any date), ~nil~ (to match items that are not schedule), ~past~ (to match items scheduled for the past), ~today~ (to match items scheduled for today), or ~future~ (to match items scheduled for the future).  Argument may also be given like ~before DATE~ or ~after DATE~ where DATE is a date string that ~org-time-string-to-absolute~ can process.
++  =:scheduled= :: Group items that are scheduled.  Argument can be ~t~ (to match items scheduled for any date), ~nil~ (to match items that are not schedule), ~past~ (to match items scheduled for the past), ~today~ (to match items scheduled for today), or ~future~ (to match items scheduled for the future).  Argument may also be given like ~before DATE~ or ~after DATE~ where DATE is a date string that ~org-read-date~ can process, inculded relative dates, like ~mon~ (closest monday in future), ~-1~ (yesterday), ~+1m~ for the next month, ~+2w~ two weeks and so on.
 +  =:tag= :: Group items that match any of the given tags.  Argument may be a string or list of strings.
 +  =:time-grid= :: Group items that appear on the time grid.
 +  =:todo= :: Group items that match any of the given TODO keywords.  Argument may be a string or list of strings, or ~t~ to match any keyword, or ~nil~ to match only non-todo items.

examples.org

@@ -240,13 +240,8 @@ Of course, you could also write that as a standard agenda command with the  [[ht
 What if you wanted to group tasks that are due before the end of the current month?  You could use something like this:
 
 #+BEGIN_SRC elisp
-  (-let* (((sec minute hour day month year dow dst utcoff) (decode-time))
-          (last-day-of-month (calendar-last-day-of-month month year))
-          (target-date
-           ;; A hack that seems to work fine.  Yay, Postel!
-           (format "%d-%02d-%02d" year month (1+ last-day-of-month)))
-          (org-super-agenda-groups
-           `((:deadline (before ,target-date))
+  (let ((org-super-agenda-groups
+           '((:deadline (before "1 +1m")) ; first day of the next month
              (:discard (:anything t)))))
     (org-todo-list))
 #+END_SRC

org-super-agenda.el

@@ -328,6 +328,12 @@ marker."
     (buffer-substring (org-entry-beginning-position)
                       (org-entry-end-position))))
 
+(defun org-super-agenda--translate-org-date (s)
+  "Return absolute time from various org supported date format,
+included relative: +1, +mon and so on..."
+  (org-time-string-to-absolute (org-read-date nil nil s))
+  )
+
 ;;;; Minor mode
 
 ;;;###autoload
@@ -481,15 +487,15 @@ DATE', where DATE is a date string that
                    (org-today))))
          (target-date (pcase (car args)
                         ((or 'before 'on 'after)
-                         (org-time-string-to-absolute (cadr args))))))
+                         (org-super-agenda--translate-org-date (cadr args))))))
   :test (org-super-agenda--when-with-marker-buffer (org-super-agenda--get-marker item)
           (let ((entry-time (org-entry-get (point) "DEADLINE")))
             (pcase (car args)
               ('t entry-time)  ; Has any deadline info
               ('nil (not entry-time))  ; Has no deadline info
               (comparison
                (when entry-time
-                 (let ((entry-time (org-time-string-to-absolute entry-time))
+                 (let ((entry-time (org-super-agenda--translate-org-date entry-time))
                        (compare-date (pcase comparison
                                        ((or 'past 'today 'future) today)
                                        ((or 'before 'on 'after) target-date))))
@@ -518,15 +524,15 @@ DATE', where DATE is a date string that
                    (org-today))))
          (target-date (pcase (car args)
                         ((or 'before 'on 'after)
-                         (org-time-string-to-absolute (cadr args))))))
+                         (org-super-agenda--translate-org-date (cadr args))))))
   :test (org-super-agenda--when-with-marker-buffer (org-super-agenda--get-marker item)
           (let ((entry-time (org-entry-get (point) "SCHEDULED")))
             (pcase (car args)
               ('t entry-time)  ; Has any scheduled info
               ('nil (not entry-time))  ; Has no scheduled info
               (comparison
                (when entry-time
-                 (let ((entry-time (org-time-string-to-absolute entry-time))
+                 (let ((entry-time (org-super-agenda--translate-org-date entry-time))
                        (compare-date (pcase comparison
                                        ((or 'past 'today 'future) today)
                                        ((or 'before 'on 'after) target-date))))