diff --git a/libraries/AP_HAL_PX4/Scheduler.cpp b/libraries/AP_HAL_PX4/Scheduler.cpp
index b01039e23b..45ea3f7523 100644
--- a/libraries/AP_HAL_PX4/Scheduler.cpp
+++ b/libraries/AP_HAL_PX4/Scheduler.cpp
@@ -38,8 +38,6 @@ PX4Scheduler::PX4Scheduler() :
 
 void PX4Scheduler::init(void *unused) 
 {
-    _sketch_start_time = hrt_absolute_time();
-
     _main_task_pid = getpid();
 
     // setup the timer thread - this will call tasks at 1kHz
@@ -78,7 +76,7 @@ void PX4Scheduler::init(void *unused)
 
 uint64_t PX4Scheduler::micros64() 
 {
-    return hrt_absolute_time() - _sketch_start_time;
+    return hrt_absolute_time();
 }
 
 uint64_t PX4Scheduler::millis64() 
@@ -116,7 +114,7 @@ void PX4Scheduler::delay_microseconds(uint16_t usec)
         perf_end(_perf_delay);
         return;
     }
-	uint64_t start = micros();
+	uint64_t start = micros64();
     uint64_t dt;
 	while ((dt=(micros64() - start)) < usec) {
 		up_udelay(usec - dt);
@@ -131,9 +129,9 @@ void PX4Scheduler::delay(uint16_t ms)
         return;
     }
     perf_begin(_perf_delay);
-	uint64_t start = hrt_absolute_time();
+	uint64_t start = micros64();
     
-    while ((hrt_absolute_time() - start)/1000 < ms && 
+    while ((micros64() - start)/1000 < ms && 
            !_px4_thread_should_exit) {
         delay_microseconds_semaphore(1000);
         if (_min_delay_cb_ms <= ms) {
diff --git a/libraries/AP_HAL_PX4/Scheduler.h b/libraries/AP_HAL_PX4/Scheduler.h
index 891eeb3456..0e40573846 100644
--- a/libraries/AP_HAL_PX4/Scheduler.h
+++ b/libraries/AP_HAL_PX4/Scheduler.h
@@ -53,7 +53,6 @@ private:
     uint16_t _min_delay_cb_ms;
     AP_HAL::Proc _failsafe;
     volatile bool _timer_pending;
-    uint64_t _sketch_start_time;
 
     volatile bool _timer_suspended;
 
diff --git a/libraries/AP_HAL_PX4/UARTDriver.cpp b/libraries/AP_HAL_PX4/UARTDriver.cpp
index fd0219381c..6985992ded 100644
--- a/libraries/AP_HAL_PX4/UARTDriver.cpp
+++ b/libraries/AP_HAL_PX4/UARTDriver.cpp
@@ -391,7 +391,7 @@ int PX4UARTDriver::_write_fd(const uint8_t *buf, uint16_t n)
             _last_write_time != 0 &&
             _total_written != 0 &&
             _os_write_buffer_size == _total_written &&
-            hrt_elapsed_time(&_last_write_time) > 500*1000UL) {
+            (hal.scheduler->micros64() - _last_write_time) > 500*1000UL) {
             // it doesn't look like hw flow control is working
             ::printf("disabling flow control on %s _total_written=%u\n", 
                      _devpath, (unsigned)_total_written);
@@ -407,17 +407,17 @@ int PX4UARTDriver::_write_fd(const uint8_t *buf, uint16_t n)
 
     if (ret > 0) {
         BUF_ADVANCEHEAD(_writebuf, ret);
-        _last_write_time = hrt_absolute_time();
+        _last_write_time = hal.scheduler->micros64();
         _total_written += ret;
         return ret;
     }
 
-    if (hrt_absolute_time() - _last_write_time > 2000 &&
+    if (hal.scheduler->micros64() - _last_write_time > 2000 &&
         _flow_control == FLOW_CONTROL_DISABLE) {
 #if 0
         // this trick is disabled for now, as it sometimes blocks on
         // re-opening the ttyACM0 port, which would cause a crash
-        if (hrt_absolute_time() - _last_write_time > 2000000) {
+        if (hal.scheduler->micros64() - _last_write_time > 2000000) {
             // we haven't done a successful write for 2 seconds - try
             // reopening the port        
             _initialised = false;
@@ -430,11 +430,11 @@ int PX4UARTDriver::_write_fd(const uint8_t *buf, uint16_t n)
                 return n;
             }
             
-            _last_write_time = hrt_absolute_time();
+            _last_write_time = hal.scheduler->micros64();
             _initialised = true;
         }
 #else
-        _last_write_time = hrt_absolute_time();
+        _last_write_time = hal.scheduler->micros64();
 #endif
         // we haven't done a successful write for 2ms, which means the 
         // port is running at less than 500 bytes/sec. Start