Some examples: local-time

Common Lisp local-time

(in-package #:local-time)

(defun get-previous-day (&key (timestamp (now)))
  "Returns the timestamp of the previous day of the timestamp"
  (adjust-timestamp timestamp (offset :day -1)))

(defun get-next-day (&key (timestamp (now)))
  "Returns the timestamp of the next day of the timestamp"
  (adjust-timestamp timestamp (offset :day 1)))

(defun get-previous-month (year month)
 "Returns a list with the year and month as number which are 
the previous month of the year."
  (let ((new-year nil)
	(new-month nil))
    (if (= month 1)
	(setf new-month 12
	      new-year (- year 1))
	
	(setf new-month (- month 1)
	      new-year year))
    (list new-year new-month)))

(defun get-next-month (year month)
  "Returns a list with the year and month as number which are
 the previous next of the year."
  (let ((new-year nil)
	(new-month nil))
    (if (= month 12)
	(setf new-month 1
	      new-year (+ 1 year))
	
	(setf new-month (+ 1 month)
	      new-year year))
    (list new-year new-month)))

(defun get-month-period (year month)
  "Returns the start and end date of the month"
  (let ((last-day (days-in-month month year)))
    (list (ucw::strcat year "/" month "/" 1)
	  (ucw::strcat year "/" month "/" last-day))))