diff --git a/src/runtime-three/runtime-npc-presence.ts b/src/runtime-three/runtime-npc-presence.ts
new file mode 100644
index 00000000..eb251393
--- /dev/null
+++ b/src/runtime-three/runtime-npc-presence.ts
@@ -0,0 +1,49 @@
+import type { NpcPresence } from "../entities/entity-instances";
+
+import {
+  hasTimeWindowJustEnded,
+  hasTimeWindowJustStarted,
+  isWithinTimeWindow
+} from "./runtime-project-time";
+
+export function resolveNpcPresenceActive(
+  presence: NpcPresence,
+  timeOfDayHours: number
+): boolean {
+  switch (presence.mode) {
+    case "always":
+      return true;
+    case "timeWindow":
+      return isWithinTimeWindow(
+        presence.startHour,
+        presence.endHour,
+        timeOfDayHours
+      );
+  }
+}
+
+export function hasNpcPresenceActivityChanged(
+  presence: NpcPresence,
+  previousTimeOfDayHours: number,
+  currentTimeOfDayHours: number
+): boolean {
+  switch (presence.mode) {
+    case "always":
+      return false;
+    case "timeWindow":
+      return (
+        hasTimeWindowJustStarted(
+          previousTimeOfDayHours,
+          currentTimeOfDayHours,
+          presence.startHour,
+          presence.endHour
+        ) ||
+        hasTimeWindowJustEnded(
+          previousTimeOfDayHours,
+          currentTimeOfDayHours,
+          presence.startHour,
+          presence.endHour
+        )
+      );
+  }
+}