diff --git a/src/lib/hafas-time.ts b/src/lib/hafas-time.ts
index 79dd32b..897ac2a 100644
--- a/src/lib/hafas-time.ts
+++ b/src/lib/hafas-time.ts
@@ -2,9 +2,14 @@
 // HAFAS returns times in Vienna (CET/CEST). This module provides correct parsing
 // regardless of the server or browser timezone.
 
-// Helper: get the offset (in minutes) from UTC for a given instant in a specific IANA timezone.
-// Reads the hour via Intl.DateTimeFormat in both UTC and the target timezone, then
-// computes the difference. Handles midnight wraparound (e.g. UTC 23:xx → TZ 01:xx).
+// Helper: get the offset (in minutes) from UTC for a given instant in a specific
+// IANA timezone.
+//
+// CAVEAT — Hour-level precision only. This helper reads the hour via
+// Intl.DateTimeFormat (hour part only), so it returns incorrect results for
+// timezones with fractional-hour offsets (e.g. India +05:30, Nepal +05:45).
+// It is designed exclusively for "Europe/Vienna" (CET/CEST), which only uses
+// full-hour offsets (+01:00 / +02:00). Do not reuse for other timezones.
 function getTimezoneOffsetMinutes(instant: Date, tz: string): number {
   const getHour = (timeZone: string) => {
     const parts = new Intl.DateTimeFormat("en-US", {