diff --git a/src/local-time.lisp b/src/local-time.lisp
index 2384f3fd..62d9597b 100644
--- a/src/local-time.lisp
+++ b/src/local-time.lisp
@@ -204,7 +204,7 @@
     (values (elt (timezone-subzones timezone) subzone-idx)
             transition-idx)))
 
-(defun %guess-offset (seconds days &optional timezone)
+(defun %guess-unix-offset (unix-time &optional timezone)
   ;; try converting the local time to a timestamp using each available
   ;; subtimezone, until we find one where the offset matches the offset that
   ;; applies at that time (according to the transition table).
@@ -213,10 +213,20 @@
   ;; Whichever subtimezone is listed first in the tzinfo database will be
   ;; the one that we pick to resolve ambiguous local time representations.
   (let* ((zone (%realize-timezone (or timezone *default-timezone*)))
-         (unix-time (timestamp-values-to-unix seconds days))
-         (subzone (%subzone-as-of zone unix-time)))
+         (subzone-idx (if (zerop (length (timezone-indexes zone)))
+                          0
+                          (elt (timezone-indexes zone)
+                               (transition-position unix-time
+                                                    (timezone-transitions zone)))))
+         (subzone (elt (timezone-subzones zone) subzone-idx)))
     (subzone-offset subzone)))
 
+(defun %guess-offset (seconds days &optional timezone)
+  ;; check that guessed offset still applies
+  (let* ((unix-time (timestamp-values-to-unix seconds days))
+         (offset (%guess-unix-offset unix-time timezone)))
+    (%guess-unix-offset (- unix-time offset) timezone)))
+
 (defun %read-binary-integer (stream byte-count &optional (signed nil))
   "Read BYTE-COUNT bytes from the binary stream STREAM, and return an integer which is its representation in network byte order (MSB).  If SIGNED is true, interprets the most significant bit as a sign indicator."
   (loop