A simple function to generate streams of dates based on certain params (uses simple-date-time Common Lisp library)

date-stream.lisp

(defun date-stream (&key (start-date nil) (interval 1) (interval-type :day))
   (let ((start (if (not start-date) (now) start-date)))
     (lambda ()
       (let ((current start))
         (setq start (funcall (cond
                                ((eq interval-type :day) #'day+)
                                ((eq interval-type :month) #'month+)
                                ((eq interval-type :year) #'year+)
                                (t #'day+)) start interval))
         current))))

;; Usage
;;DATE-TEST> (defparameter *ds* (date-stream :interval 10))
;;*DS*
;;DATE-TEST> (funcall *ds*)
;;#<DATE-TIME 2019-06-26 21:55:26.515 {1005E4D3B3}>
;;DATE-TEST> (funcall *ds*)
;;#<DATE-TIME 2019-07-06 21:55:26.515 {1005E69D63}>
;;DATE-TEST> (funcall *ds*)
;;#<DATE-TIME 2019-07-16 21:55:26.515 {1005E6EA03}>

;;DATE-TEST> (defparameter *ds2* (date-stream :interval 1 :interval-type :month))
;;*DS2*
;;DATE-TEST> (funcall *ds2*)
;;#<DATE-TIME 2019-06-26 22:01:38.198 {100667B753}>
;;DATE-TEST> (funcall *ds2*)
;;#<DATE-TIME 2019-07-26 22:01:38.198 {100667C523}>

;;DATE-TEST> (defparameter *ds2* (date-stream :start-date (day+ (now) 4) :interval-type :month))
;;*DS2*
;;DATE-TEST> (funcall *ds2*)
;;#<DATE-TIME 2019-06-30 22:05:09.301 {1002783B93}>
;; Call lots of time till February
;;DATE-TEST> (funcall *ds2*)
;;#<DATE-TIME 2020-02-29 22:05:09.301 {100278AE13}>