diff --git a/src/Std/Time/Zoned/ZoneRules.lean b/src/Std/Time/Zoned/ZoneRules.lean
index 72253c6d05f2..e196a3f84dbb 100644
--- a/src/Std/Time/Zoned/ZoneRules.lean
+++ b/src/Std/Time/Zoned/ZoneRules.lean
@@ -148,7 +148,7 @@ If the timestamp falls between two transitions, it returns the most recent trans
 def findTransitionIndexForTimestamp (transitions : Array Transition) (timestamp : Timestamp) : Option Nat :=
   let value := timestamp.toSecondsSinceUnixEpoch
   if let some idx := transitions.findIdx? (fun t => t.time.val ≥ value.val)
-    then some (idx - 1)
+    then some idx
     else none
 
 /--