diff --git a/ion/include/ion/timing.h b/ion/include/ion/timing.h
index 8b99901bb..1d077ab93 100644
--- a/ion/include/ion/timing.h
+++ b/ion/include/ion/timing.h
@@ -9,7 +9,7 @@ void usleep(uint32_t us);
 #define EXTERNC
 #endif
 
-EXTERNC uint32_t millis();
+EXTERNC uint64_t millis();
 
 EXTERNC void msleep(uint32_t ms);
 
diff --git a/ion/src/device/boot/rt0.cpp b/ion/src/device/boot/rt0.cpp
index b9734b7f3..332ab5301 100644
--- a/ion/src/device/boot/rt0.cpp
+++ b/ion/src/device/boot/rt0.cpp
@@ -93,7 +93,7 @@ void start() {
   abort();
 }
 
-volatile uint32_t millis_elapsed = 0;
+volatile uint64_t millis_elapsed = 0;
 
 void __attribute__((interrupt)) sysTick() {
   millis_elapsed++;
diff --git a/ion/src/device/device.cpp b/ion/src/device/device.cpp
index cb18bf0d8..a53d851c7 100644
--- a/ion/src/device/device.cpp
+++ b/ion/src/device/device.cpp
@@ -39,9 +39,9 @@ void Ion::usleep(uint32_t us) {
   }
 }
 
-extern volatile uint32_t millis_elapsed;
+extern volatile uint64_t millis_elapsed;
 
-uint32_t Ion::millis() {
+uint64_t Ion::millis() {
   return millis_elapsed;
 }
 
diff --git a/ion/src/shared/timing.cpp b/ion/src/shared/timing.cpp
index a39d450d6..6e47a293e 100644
--- a/ion/src/shared/timing.cpp
+++ b/ion/src/shared/timing.cpp
@@ -3,7 +3,7 @@
 
 static auto start = std::chrono::steady_clock::now();
 
-uint32_t Ion::millis() {
+uint64_t Ion::millis() {
   auto elapsed = std::chrono::steady_clock::now() - start;
   return std::chrono::duration_cast<std::chrono::milliseconds>(elapsed).count();
 }