diff --git a/driver/panda_stream.c b/driver/panda_stream.c
index 626762d..e51353b 100644
--- a/driver/panda_stream.c
+++ b/driver/panda_stream.c
@@ -80,6 +80,8 @@ struct stream_open {
     bool stream_active;         // If not set, any interrupts are unexpected
     uint32_t completion;        // Copy of final interrupt status register
     struct timespec64 start_ts; // timestamp when PCAP becomes armed & enabled
+    bool start_ts_valid;        // whether start_ts was set
+    spinlock_t start_ts_lock;   // it handles race between isr and ioctl
 
     /* Reader status. */
     int read_block_index;       // Block being read
@@ -201,8 +203,13 @@ static irqreturn_t stream_isr(int irq, void *dev_id)
 
     if (open->stream_active)
     {
-        if (IRQ_STATUS_START_EVENT(status))
+        if (IRQ_STATUS_START_EVENT(status)) {
+            unsigned long flags;
+            spin_lock_irqsave(&open->start_ts_lock, flags);
             ktime_get_real_ts64(&open->start_ts);
+            open->start_ts_valid = true;
+            spin_unlock_irqrestore(&open->start_ts_lock, flags);
+        }
 
         if (IRQ_STATUS_NEW_BUFFER(status))
         {
@@ -297,8 +304,9 @@ static void start_hardware(struct stream_open *open)
     open->isr_block_index = 0;
     open->read_block_index = 0;
     open->read_offset = 0;
-    /* having a zeroed timestamp means the start event did not happen. */
     open->start_ts = (const struct timespec64){0};
+    open->start_ts_valid = false;
+    spin_lock_init(&open->start_ts_lock);
     /* After this point we can allow interrupts, they can potentially start as
      * soon as a DMA buffer is assigned. */
     smp_wmb();
@@ -515,9 +523,20 @@ static long panda_stream_ioctl(
         case PANDA_COMPLETION:
             return stream_completion(open, (void __user *) arg);
         case PANDA_GET_START_TS:
-            return copy_to_user(
-                (void __user *) arg, &open->start_ts,
-                sizeof(struct timespec64)) ? -EIO : 0;
+            {
+                struct timespec64 ts;
+                bool ts_valid;
+                spin_lock(&open->start_ts_lock);
+                ts = open->start_ts;
+                ts_valid = open->start_ts_valid;
+                spin_unlock(&open->start_ts_lock);
+                if (!ts_valid)
+                    return -EAGAIN;
+
+                return copy_to_user(
+                    (void __user *) arg, &ts,
+                    sizeof(struct timespec64)) ? -EIO : 0;
+            }
         default:
             return -EINVAL;
     }
diff --git a/server/data_server.c b/server/data_server.c
index e5660e0..37bba82 100644
--- a/server/data_server.c
+++ b/server/data_server.c
@@ -113,10 +113,13 @@ static uint64_t experiment_sample_count;
 /* Save start timestamp, either from hardware (if that timestamp is not 0), or
  * from the driver (which was saved in the interrupt handler triggered by the
  * start event) */
-static void update_start_timestamp(void)
+static bool update_start_timestamp(void)
 {
     struct timespec pcap_drv_start_ts, pcap_hw_start_ts;
-    hw_get_start_ts(&pcap_drv_start_ts);
+    bool valid_ts = hw_get_start_ts(&pcap_drv_start_ts);
+    if (!valid_ts)
+        return false;
+
     hw_get_hw_start_ts(&pcap_hw_start_ts);
     if (pcap_hw_start_ts.tv_sec == 0 && pcap_hw_start_ts.tv_nsec == 0)
     {
@@ -133,6 +136,8 @@ static void update_start_timestamp(void)
         pcap_hw_ts_offset_ns = drv_ts_num - hw_ts_num;
         pcap_hw_ts_offset_ns_valid = true;
     }
+
+    return true;
 }
 
 
@@ -154,18 +159,17 @@ static void capture_experiment(void)
     {
         void *block = get_write_block(data_buffer);
         size_t count;
-        do
+        do {
             count = hw_read_streamed_data(block, DATA_BLOCK_SIZE, &at_eof);
-        while (data_thread_running  &&  count == 0  &&  !at_eof);
-
-        /* Do our best to capture a timestamp before releasing the first block
-         * of the experiment.  If the experiment is empty this return an empty
-         * timestamp, that's how it goes. */
-        if (!ts_captured)
-        {
-            update_start_timestamp();
-            ts_captured = true;
-        }
+            if (!ts_captured)
+            {
+                ts_captured = update_start_timestamp();
+                /* we want to release the block when we get the timestamp to
+                 * notify the readers */
+                if (ts_captured)
+                    break;
+            }
+        } while (data_thread_running  &&  count == 0  &&  !at_eof);
 
         /* Unconditionally release the write block here.  Either we have data to
          * send (exited loop above with count==0), or we're at the end of the
diff --git a/server/hardware.c b/server/hardware.c
index e0ebc38..73a23e7 100644
--- a/server/hardware.c
+++ b/server/hardware.c
@@ -316,22 +316,34 @@ unsigned int hw_read_streamed_completion(void)
 }
 
 
-void hw_get_start_ts(struct timespec *ts)
+bool hw_get_start_ts(struct timespec *ts)
 {
-    struct timespec64 compat_ts = (struct timespec64) {0};
-    error_report(TEST_IO(ioctl(stream, PANDA_GET_START_TS, &compat_ts)));
-    ts->tv_sec = (time_t) compat_ts.tv_sec;
-    ts->tv_nsec = (typeof(ts->tv_nsec)) compat_ts.tv_nsec;
+    struct timespec64 compat_ts = {};
+    if (ioctl(stream, PANDA_GET_START_TS, &compat_ts) == -1)
+    {
+        // EAGAIN indicates the timestamp hasn't been captured yet
+        if (errno != EAGAIN)
+            error_report(TEST_IO(-1));
+
+        return false;
+    }
+    else
+    {
+        ts->tv_sec = (time_t) compat_ts.tv_sec;
+        ts->tv_nsec = (typeof(ts->tv_nsec)) compat_ts.tv_nsec;
+        return true;
+    }
 }
 
 
-void hw_get_hw_start_ts(struct timespec *ts)
+bool hw_get_hw_start_ts(struct timespec *ts)
 {
     ts->tv_sec = (time_t) read_named_register(PCAP_TS_SEC);
     ts->tv_nsec = (typeof(ts->tv_nsec)) ((uint64_t)
         read_named_register(PCAP_TS_TICKS) * NSECS / hw_read_nominal_clock());
     ts->tv_sec += ts->tv_nsec / NSECS;
     ts->tv_nsec = ts->tv_nsec % NSECS;
+    return true;
 }
 
 #endif
diff --git a/server/hardware.h b/server/hardware.h
index 2ee761b..82c84f9 100644
--- a/server/hardware.h
+++ b/server/hardware.h
@@ -165,9 +165,9 @@ unsigned int hw_read_streamed_completion(void);
 const char *hw_decode_completion(unsigned int completion);
 
 /* This function gets the timestamp when PCAP becomes armed and enabled */
-void hw_get_start_ts(struct timespec *ts);
+bool hw_get_start_ts(struct timespec *ts);
 /* This one is latched in hardware instead of the driver */
-void hw_get_hw_start_ts(struct timespec *ts);
+bool hw_get_hw_start_ts(struct timespec *ts);
 
 /* This function controls the arm/disarm state of data capture.  Data capture is
  * armed by writing true with this function, after which hw_read_streamed_data()
diff --git a/server/sim_hardware.c b/server/sim_hardware.c
index 2450778..47ab1a4 100644
--- a/server/sim_hardware.c
+++ b/server/sim_hardware.c
@@ -182,16 +182,18 @@ void hw_write_arm_streamed_data(void) { }
 uint32_t hw_read_streamed_completion(void) { return 0; }
 
 
-void hw_get_start_ts(struct timespec *ts)
+bool hw_get_start_ts(struct timespec *ts)
 {
     clock_gettime(CLOCK_REALTIME, ts);
+    return true;
 }
 
 
-void hw_get_hw_start_ts(struct timespec *ts)
+bool hw_get_hw_start_ts(struct timespec *ts)
 {
     ts->tv_sec = 0;
     ts->tv_nsec = 0;
+    return true;
 }