diff --git a/src/app/api/medications/[id]/route.ts b/src/app/api/medications/[id]/route.ts
index ca2ec4e7a..8bd2587af 100644
--- a/src/app/api/medications/[id]/route.ts
+++ b/src/app/api/medications/[id]/route.ts
@@ -32,6 +32,7 @@ import {
   recomputeMedicationComplianceForDay,
 } from "@/lib/rollups/medication-compliance-rollups";
 import { assertMedicationOwnership } from "@/lib/medications/route-guards";
+import { hhmmToMinutesOrNull } from "@/lib/medications/scheduling/hhmm";
 import { getUserTodayBounds } from "@/lib/tz/local-day";
 import { NextRequest } from "next/server";
 
@@ -39,9 +40,7 @@ type RouteParams = { params: Promise<{ id: string }> };
 
 /** Parse "HH:mm" into minutes-of-day; NaN-safe (malformed → 0). */
 function hhmmToMinutes(value: string): number {
-  const [h, m] = value.split(":").map(Number);
-  if (!Number.isFinite(h) || !Number.isFinite(m)) return 0;
-  return h * 60 + m;
+  return hhmmToMinutesOrNull(value) ?? 0;
 }
 
 /**
diff --git a/src/components/medications/scheduling/dose-window.ts b/src/components/medications/scheduling/dose-window.ts
index c6caf558b..a468feb1b 100644
--- a/src/components/medications/scheduling/dose-window.ts
+++ b/src/components/medications/scheduling/dose-window.ts
@@ -21,6 +21,11 @@
  */
 
 import { DOSE_WINDOW_DEFAULTS } from "@/lib/medications/scheduling/dose-window-defaults";
+import { hhmmToMinutes } from "@/lib/medications/scheduling/hhmm";
+
+// Re-exported so the existing `dose-window` consumers (and its unit suite)
+// keep their import site while the implementation lives in the shared util.
+export { hhmmToMinutes };
 
 /** One persisted explicit window. Matches `doseWindowEntrySchema`. */
 export interface DoseWindowEntry {
@@ -40,12 +45,6 @@ export type DoseWindowScale = "intraday" | "dayScale";
 
 const TIME_RE = /^([01]\d|2[0-3]):([0-5]\d)$/;
 
-/** Minutes-since-midnight for a regex-valid HH:mm. */
-export function hhmmToMinutes(hhmm: string): number {
-  const [h, m] = hhmm.split(":");
-  return Number(h) * 60 + Number(m);
-}
-
 /** Wrap minutes-since-midnight back to HH:mm (clamps into the day, 24h wrap). */
 export function minutesToHhmm(total: number): string {
   const wrapped = ((total % 1440) + 1440) % 1440;
diff --git a/src/lib/medications/scheduling/cadence.ts b/src/lib/medications/scheduling/cadence.ts
index 6cfa837be..d4d6bf0cf 100644
--- a/src/lib/medications/scheduling/cadence.ts
+++ b/src/lib/medications/scheduling/cadence.ts
@@ -46,6 +46,7 @@ import {
   occurrencesBetween,
 } from "@/lib/medications/scheduling/recurrence";
 import { normaliseDoseWindows } from "@/lib/medications/scheduling/worker-helpers";
+import { hhmmToMinutesOrNull } from "@/lib/medications/scheduling/hhmm";
 import { wallClockInTz } from "@/lib/tz/wall-clock";
 import { startOfLocalDayInTz } from "@/lib/tz/local-day";
 
@@ -325,13 +326,6 @@ function applyTime(day: Date, hhmm: string, tz: string | undefined): Date {
 }
 
 /** Parse "HH:mm" to minutes-since-midnight, or null when malformed. */
-function hhmmToMinutes(hhmm: string): number | null {
-  const [hStr, mStr] = hhmm.split(":");
-  const h = Number(hStr);
-  const m = Number(mStr);
-  if (!Number.isFinite(h) || !Number.isFinite(m)) return null;
-  return h * 60 + m;
-}
 
 /** Snap a Date down to the user-local midnight. */
 export function startOfLocalDay(d: Date, tz: string | undefined): Date {
@@ -504,8 +498,8 @@ export function expandScheduleSlots(
       // the windowStart..windowEnd span so the pairing radius and the
       // chart cell keep their existing shape; an overnight window pushes
       // the end to the next day.
-      const startMin = hhmmToMinutes(schedule.windowStart);
-      const endMin = hhmmToMinutes(schedule.windowEnd);
+      const startMin = hhmmToMinutesOrNull(schedule.windowStart);
+      const endMin = hhmmToMinutesOrNull(schedule.windowEnd);
       let spanMs = DAY_MS;
       if (startMin !== null && endMin !== null) {
         let span = endMin - startMin;
diff --git a/src/lib/medications/scheduling/hhmm.ts b/src/lib/medications/scheduling/hhmm.ts
new file mode 100644
index 000000000..c2aeaf415
--- /dev/null
+++ b/src/lib/medications/scheduling/hhmm.ts
@@ -0,0 +1,38 @@
+/**
+ * Shared `HH:mm` → minutes-since-midnight helpers.
+ *
+ * Consolidates the copies that had accumulated across the medication
+ * scheduling surfaces (the window validator, the dose-window geometry, the
+ * per-medication schedule reconciler, the slot-instant resolver, the
+ * recurrence engine, the cadence walker, and the worker helpers). Two
+ * variants, matching the two behaviours those copies actually relied on:
+ *
+ *   - `hhmmToMinutes` assumes the literal was already regex-validated (the
+ *     Zod window schema, a persisted schedule row) and does the arithmetic
+ *     directly — a malformed literal yields `NaN`, which never reaches these
+ *     callers;
+ *   - `hhmmToMinutesOrNull` handles unvalidated input and returns `null` when
+ *     the literal is not a well-formed numeric `HH:mm`, so a caller can decide
+ *     the fallback (skip the time, treat as 0, …) explicitly.
+ *
+ * Pure string arithmetic, no imports — safe to pull into both server and
+ * client bundles.
+ */
+
+/** Minutes-since-midnight for an already-validated `HH:mm` literal. */
+export function hhmmToMinutes(hhmm: string): number {
+  const [h, m] = hhmm.split(":");
+  return Number(h) * 60 + Number(m);
+}
+
+/**
+ * Minutes-since-midnight, or `null` when the literal is not a well-formed
+ * numeric `HH:mm`. Use on any path that handles unvalidated schedule input.
+ */
+export function hhmmToMinutesOrNull(hhmm: string): number | null {
+  const [hStr, mStr] = hhmm.split(":");
+  const h = Number(hStr);
+  const m = Number(mStr);
+  if (!Number.isFinite(h) || !Number.isFinite(m)) return null;
+  return h * 60 + m;
+}
diff --git a/src/lib/medications/scheduling/recurrence.ts b/src/lib/medications/scheduling/recurrence.ts
index 5a6b9ae8b..f05322d56 100644
--- a/src/lib/medications/scheduling/recurrence.ts
+++ b/src/lib/medications/scheduling/recurrence.ts
@@ -60,6 +60,7 @@ import { annotate } from "@/lib/logging/context";
 import { parseScheduleRecurrence } from "@/lib/medication-schedule";
 import { wallClockInTz } from "@/lib/tz/wall-clock";
 import { startOfLocalDayInTz } from "@/lib/tz/local-day";
+import { hhmmToMinutesOrNull } from "@/lib/medications/scheduling/hhmm";
 
 /**
  * v1.7.0 — schedule-type discriminator on the canonical schedule.
@@ -823,8 +824,8 @@ function graceWindowMs(schedule: CanonicalSchedule): number {
   if (schedule.reminderGraceMinutes !== null) {
     return schedule.reminderGraceMinutes * ONE_MINUTE_MS;
   }
-  const startMin = hhmmToMinutes(schedule.windowStart);
-  const endMin = hhmmToMinutes(schedule.windowEnd);
+  const startMin = hhmmToMinutesOrNull(schedule.windowStart);
+  const endMin = hhmmToMinutesOrNull(schedule.windowEnd);
   if (startMin === null || endMin === null) {
     return DEFAULT_GRACE_MINUTES * ONE_MINUTE_MS;
   }
@@ -834,14 +835,6 @@ function graceWindowMs(schedule: CanonicalSchedule): number {
   return span * ONE_MINUTE_MS;
 }
 
-function hhmmToMinutes(hhmm: string): number | null {
-  const [hStr, mStr] = hhmm.split(":");
-  const h = Number(hStr);
-  const m = Number(mStr);
-  if (!Number.isFinite(h) || !Number.isFinite(m)) return null;
-  return h * 60 + m;
-}
-
 // ────────────────────────────────────────────────────────────────────
 // UTC date helpers (RRULE day-anchor + endsOn cap arithmetic)
 // ────────────────────────────────────────────────────────────────────
diff --git a/src/lib/medications/scheduling/resolve-slot-instant.ts b/src/lib/medications/scheduling/resolve-slot-instant.ts
index d53fa52e2..56273fbd8 100644
--- a/src/lib/medications/scheduling/resolve-slot-instant.ts
+++ b/src/lib/medications/scheduling/resolve-slot-instant.ts
@@ -33,6 +33,7 @@ import {
   type CanonicalSchedule,
   type RecurrenceContext,
 } from "@/lib/medications/scheduling/recurrence";
+import { hhmmToMinutesOrNull } from "@/lib/medications/scheduling/hhmm";
 import { localHmAsUtc } from "@/lib/tz/local-day";
 import { wallClockInTz } from "@/lib/tz/wall-clock";
 
@@ -238,7 +239,7 @@ function canonicalSlotInstant(
   timeOfDay: string,
   userTz: string,
 ): Date {
-  const minutes = hhmmToMinutes(timeOfDay);
+  const minutes = hhmmToMinutesOrNull(timeOfDay);
   if (minutes === null) return occAt;
   const h = Math.floor(minutes / 60);
   const m = minutes % 60;
@@ -267,8 +268,8 @@ function canonicalSlotInstant(
  * half-span behaviour unchanged.
  */
 function snapToleranceMs(schedule: CanonicalSchedule): number {
-  const startMin = hhmmToMinutes(schedule.windowStart);
-  const endMin = hhmmToMinutes(schedule.windowEnd);
+  const startMin = hhmmToMinutesOrNull(schedule.windowStart);
+  const endMin = hhmmToMinutesOrNull(schedule.windowEnd);
   let halfSpanMs: number;
   if (startMin === null || endMin === null) {
     halfSpanMs = DEFAULT_TOLERANCE_MS;
@@ -281,7 +282,7 @@ function snapToleranceMs(schedule: CanonicalSchedule): number {
   let toleranceMs = halfSpanMs;
 
   const slots = effectiveTimesOfDay(schedule)
-    .map(hhmmToMinutes)
+    .map(hhmmToMinutesOrNull)
     .filter((m): m is number => m !== null)
     .sort((a, b) => a - b);
   if (slots.length > 1) {
@@ -325,7 +326,7 @@ function graceToleranceMs(schedule: CanonicalSchedule): number {
       : ONE_MINUTE_MS;
 
   const slots = effectiveTimesOfDay(schedule)
-    .map(hhmmToMinutes)
+    .map(hhmmToMinutesOrNull)
     .filter((m): m is number => m !== null)
     .sort((a, b) => a - b);
   if (slots.length > 1) {
@@ -353,11 +354,3 @@ function effectiveTimesOfDay(schedule: CanonicalSchedule): string[] {
     ? schedule.timesOfDay
     : [schedule.windowStart];
 }
-
-function hhmmToMinutes(hhmm: string): number | null {
-  const [hStr, mStr] = hhmm.split(":");
-  const h = Number(hStr);
-  const m = Number(mStr);
-  if (!Number.isFinite(h) || !Number.isFinite(m)) return null;
-  return h * 60 + m;
-}
diff --git a/src/lib/medications/scheduling/worker-helpers.ts b/src/lib/medications/scheduling/worker-helpers.ts
index fa66b78ed..92e18ffa9 100644
--- a/src/lib/medications/scheduling/worker-helpers.ts
+++ b/src/lib/medications/scheduling/worker-helpers.ts
@@ -23,6 +23,7 @@ import {
   type ScheduleType,
   occurrencesBetween,
 } from "@/lib/medications/scheduling/recurrence";
+import { hhmmToMinutes } from "@/lib/medications/scheduling/hhmm";
 
 /**
  * Minimal Prisma-shape projection used by the worker. Mirrors the
@@ -122,11 +123,6 @@ export function normaliseDoseWindows(raw: unknown): DoseWindowEntry[] | null {
   return out.length > 0 ? out : null;
 }
 
-function hhmmToMinutes(hhmm: string): number {
-  const [h, m] = hhmm.split(":");
-  return Number(h) * 60 + Number(m);
-}
-
 /** Build the canonical engine context from worker-loop state. */
 export function buildRecurrenceContext(input: {
   medication: WorkerMedicationRow;
diff --git a/src/lib/validations/medication/base.ts b/src/lib/validations/medication/base.ts
index 7a6709c5c..b1da554a4 100644
--- a/src/lib/validations/medication/base.ts
+++ b/src/lib/validations/medication/base.ts
@@ -2,6 +2,7 @@ import { z } from "zod/v4";
 
 import { INJECTION_SITE_KEYS } from "@/lib/medications/injection-sites";
 import { validateEntryInstant } from "@/lib/validations/entry-instant";
+import { hhmmToMinutes } from "@/lib/medications/scheduling/hhmm";
 
 /**
  * v1.8.5 — the eight injection-site enum values, mirrored from the
@@ -190,12 +191,6 @@ export const doseWindowEntrySchema = z
       "One explicit per-dose on-time intake window. `timeOfDay` matches a schedule dose time; `[start, end]` (HH:mm, user local, `start <= end`) is the on-time band. Outside it the cadence-derived late tail applies, then ad-hoc.",
   });
 
-/** Minutes-since-midnight for an `HH:mm` literal (already regex-validated). */
-export function hhmmToMinutes(hhmm: string): number {
-  const [h, m] = hhmm.split(":");
-  return Number(h) * 60 + Number(m);
-}
-
 /**
  * v1.15.19 introduced `takenAt` plausibility bounds on the EDIT path
  * (audit P0-4); v1.16.9 lifts them onto every CREATE path too (the