diff --git a/app.c b/app.c
index 548c053..9f575ec 100644
--- a/app.c
+++ b/app.c
@@ -2,6 +2,7 @@
 #include "app.h"
 #include "camera.h"
 #include "entity.h"
+#include "gametime.h"
 #include "input.h"
 #include "particle.h"
 #include "physics.h"
@@ -25,6 +26,7 @@ App *app_NewApp() {
 	app->entity   = entity_NewSystem(app);
 	app->camera   = camera_NewSystem(app);
 	app->particle = particle_NewSystem(app);
+	app->time     = gametime_NewSystem(app);
 
 	app->switch_level = NULL;
 	app->clear_color  = 0;
@@ -51,6 +53,8 @@ void app_DeleteApp(App *app) {
 	physics_DeleteSystem(app->physics);
 	player_DeleteSystem(app->player);
 	camera_DeleteSystem(app->camera);
+	particle_DeleteSystem(app->particle);
+	gametime_DeleteSystem(app->time);
 
 	free(app);
 }
@@ -64,6 +68,7 @@ void app_Advance(App *app, Duration deltaTime) {
 	if (app->switch_level != NULL)
 		_app_SwitchLevel(app);
 
+	gametime_Advance(app->time, deltaTime);
 	particle_Advance(app->particle, deltaTime);
 	input_Advance(app->input);
 	player_Advance(app->player, deltaTime);
diff --git a/app.h b/app.h
index e73f9fc..a0ee901 100644
--- a/app.h
+++ b/app.h
@@ -1,6 +1,7 @@
 #pragma once
 
 #include "entity.h"
+#include "gametime.h"
 #include "physics.h"
 #include "player.h"
 #include "input.h"
@@ -23,6 +24,7 @@ typedef struct _App {
 	System_Entity   *entity;
 	System_Camera   *camera;
 	System_Particle *particle;
+	System_GameTime *time;
 
 	char    *switch_level;
 	uint32_t clear_color;
diff --git a/gametime.c b/gametime.c
new file mode 100644
index 0000000..73d744a
--- /dev/null
+++ b/gametime.c
@@ -0,0 +1,36 @@
+
+#include "gametime.h"
+#include "types.h"
+
+
+System_GameTime *gametime_NewSystem(App *super) {
+	System_GameTime *sys = zero_malloc(sizeof(System_GameTime));
+
+	sys->super = super;
+	sys->now   = GAMETIME_EPOCH;
+	return sys;
+}
+void gametime_DeleteSystem(System_GameTime *sys) {
+	free(sys);
+}
+
+void gametime_Advance(System_GameTime *sys, Duration deltaTime) {
+	sys->now += deltaTime.microseconds;
+}
+
+
+TimePoint gametime_Now(System_GameTime *sys) {
+	TimePoint now = {.microseconds = sys->now};
+	return now;
+}
+
+Duration gametime_Since(System_GameTime *sys, TimePoint prev) {
+	return time_Difference(gametime_Now(sys), prev);
+}
+
+Duration gametime_Reset(System_GameTime *sys, TimePoint *prev) {
+	TimePoint now = gametime_Now(sys);
+	Duration  d   = time_Difference(now, *prev);
+	*prev         = now;
+	return d;
+}
diff --git a/gametime.h b/gametime.h
new file mode 100644
index 0000000..f10587f
--- /dev/null
+++ b/gametime.h
@@ -0,0 +1,40 @@
+#pragma once
+
+#include "types.h"
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
+typedef struct _App App;
+
+
+// GameTime starts counting at 100 seconds.
+#define GAMETIME_EPOCH ((int64_t)(100 * 1000 * 1000))
+
+
+// GameTime keeps track of running (or paused) time of the game.
+typedef struct {
+	App    *super;
+	int64_t now; // Microseconds since object creation, adding GAMETIME_EPOCH.
+} System_GameTime;
+
+
+System_GameTime *gametime_NewSystem(App *super);
+void             gametime_DeleteSystem(System_GameTime *sys);
+
+// Advance pushes the clock forwards by DeltaTime.
+void gametime_Advance(System_GameTime *sys, Duration deltaTime);
+
+
+// time_XXX functions, ported to GameTime.
+TimePoint gametime_Now(System_GameTime *sys);
+Duration  gametime_Since(System_GameTime *sys, TimePoint prev);
+Duration  gametime_Reset(System_GameTime *sys, TimePoint *prev);
+
+
+#ifdef __cplusplus
+}
+#endif
diff --git a/types.h b/types.h
index 32cdec3..9b76c29 100644
--- a/types.h
+++ b/types.h
@@ -107,11 +107,15 @@ typedef struct {
 	int64_t microseconds;
 } TimePoint;
 
-TimePoint time_Now();
+// No game logic should use the time_XXX functions for time.
+// Use gametime_XXX instead.
+TimePoint time_Now(); // Don't use this!
 TimePoint time_After(TimePoint now, Duration after);
-Duration  time_Since(TimePoint prev);
-Duration  time_Difference(TimePoint now, TimePoint prev);
-Duration  time_Reset(TimePoint *prev);
+// Don't use this!
+Duration time_Since(TimePoint prev);
+Duration time_Difference(TimePoint now, TimePoint prev);
+// Don't use this!
+Duration time_Reset(TimePoint *prev);
 
 // 1e-6
 extern const double EPS;