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diff --git a/module/vcomponent/timezone.scm b/module/vcomponent/timezone.scm
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+(define-module (vcomponent timezone)
+  :use-module (vcomponent)
+  :use-module ((srfi srfi-1) :select (find))
+  :use-module (srfi srfi-19)
+  :use-module (srfi srfi-19 util)
+  :use-module (srfi srfi-41)
+  :use-module (srfi srfi-41 util)
+  :use-module (util)
+  :use-module ((vcomponent recurrence generate) :select (generate-recurrence-set))
+  :use-module ((vcomponent datetime) :select (ev-time<?))
+  )
+
+;;@begin exampe
+;; <VTIMEZONE> :: "#<vcomponent 558c5da80fc0>"
+;;                  TZID: Europe/Stockholm
+;;        X-LIC-LOCATION: Europe/Stockholm
+;; : <DAYLIGHT> :: "#<vcomponent 558c5e11e7c0>"
+;; :                RRULE: FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU
+;; :              DTSTART: 19700329T020000
+;; :               TZNAME: CEST
+;; :           TZOFFSETTO: +0200
+;; :         TZOFFSETFROM: +0100
+;; : <STANDARD> :: "#<vcomponent 558c5e11e7e0>"
+;; :                RRULE: FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU
+;; :              DTSTART: 19701025T030000
+;; :               TZNAME: CET
+;; :           TZOFFSETTO: +0100
+;; :         TZOFFSETFROM: +0200
+;; @end example
+
+;; Given a tz stream of length 2, takes the time difference between the DTSTART
+;; of those two. And creates a new VTIMEZONE with that end time.
+;; TODO set remaining properties, and type of the newly created component.
+(define (extrapolate-tz-stream strm)
+  (let ((nevent (copy-vcomponent (stream-ref strm 1))))
+    (mod! (attr nevent 'DTSTART)
+          = (add-duration (time-difference
+                           (attr (stream-ref strm 1) 'DTSTART)
+                           (attr (stream-ref strm 0) 'DTSTART))))
+    (stream-append strm (stream nevent))))
+
+;; The RFC requires that at least one DAYLIGHT or STANDARD component is present.
+;; Any number of both can be present. This should handle all these cases well,
+;; as long as noone has multiple overlapping timezones, which depend on some
+;; further condition. That feels like something that should be impossible, but
+;; this is (human) time we are talking about.
+(define-public (make-tz-set tz)
+  (let ((strm (interleave-streams
+               ev-time<?
+               ;; { DAYLIGHT, STANDARD }
+               (map generate-recurrence-set (children tz)))))
+
+    (cond [(stream-null? strm) stream-null]
+
+          [(stream-null? (stream-drop 2 strm))
+           (let ((strm (extrapolate-tz-stream strm)))
+             (stream-zip strm (stream-cdr strm)))]
+
+          [else (stream-zip strm (stream-cdr strm))])))
+
+(define (parse-offset str)
+  (let* (((pm h1 h0 m1 m0) (string->list str)))
+    ((primitive-eval (symbol pm))
+     (+ (* 60 (string->number (list->string (list m1 m0))))
+        (* 60 60 (string->number (list->string (list h1 h0))))))))
+
+;; Finds the VTIMEZONE with id @var{tzid} in calendar.
+;; Crashes on error.
+(define (find-tz cal tzid)
+  (let ((ret (find (lambda (tz) (string=? tzid (attr tz 'TZID)))
+                   (children cal 'VTIMEZONE))))
+    ret))
+
+;; Takes a VEVENT.
+;; Assumes that DTSTART has a TZID property, and that that TZID is available as
+;; a direct child of the parent of @var{ev}.
+(define-public (get-tz-offset ev)
+  (let ((ret (stream-find
+              (lambda (z)
+                (let* (((start end) (map (extract 'DTSTART) z)))
+                  (and (time<=? start (attr ev 'DTSTART))
+                       (time<? (attr ev 'DTSTART) end))))
+              (attr (find-tz (parent ev)
+                             (car (prop (attr* ev 'DTSTART) 'TZID)))
+                    'X-HNH-TZSET))))
+    (if (not ret)
+        0 (parse-offset (attr (car ret) 'TZOFFSETTO)))))
+