diff --git a/.travis/script.sh b/.travis/script.sh
index 897fca2be..404f7391b 100755
--- a/.travis/script.sh
+++ b/.travis/script.sh
@@ -458,6 +458,8 @@ python --version || die
         --disable-tests || die
     make -j 8 || die
 
+    export CPPFLAGS=-DOPENTHREAD_CONFIG_ENABLE_TIME_SYNC=1
+
     git checkout -- . || die
     git clean -xfd || die
     ./bootstrap || die
diff --git a/examples/platforms/posix/radio.c b/examples/platforms/posix/radio.c
index 5f7494b02..2308f10d0 100644
--- a/examples/platforms/posix/radio.c
+++ b/examples/platforms/posix/radio.c
@@ -610,7 +610,7 @@ void radioSendMessage(otInstance *aInstance)
     if (sTransmitFrame.mIeInfo->mTimeIeOffset != 0)
     {
         uint8_t *timeIe = sTransmitFrame.mPsdu + sTransmitFrame.mIeInfo->mTimeIeOffset;
-        uint64_t time   = otPlatTimeGet() + sTransmitFrame.mIeInfo->mNetworkTimeOffset;
+        uint64_t time   = (uint64_t)((int64_t)otPlatTimeGet() + sTransmitFrame.mIeInfo->mNetworkTimeOffset);
 
         *timeIe = sTransmitFrame.mIeInfo->mTimeSyncSeq;
 
diff --git a/include/openthread/network_time.h b/include/openthread/network_time.h
index 83adaa2f5..2990c2c98 100644
--- a/include/openthread/network_time.h
+++ b/include/openthread/network_time.h
@@ -36,6 +36,7 @@
 #define OPENTHREAD_NETWORK_TIME_H_
 
 #include <openthread/error.h>
+#include <openthread/instance.h>
 
 #ifdef __cplusplus
 extern "C" {
diff --git a/src/core/mac/mac.cpp b/src/core/mac/mac.cpp
index 99a31fe10..ffe8c45c6 100644
--- a/src/core/mac/mac.cpp
+++ b/src/core/mac/mac.cpp
@@ -2389,7 +2389,7 @@ uint8_t Mac::GetTimeIeOffset(Frame &aFrame)
     VerifyOrExit(cur != NULL);
 
     cur += sizeof(VendorIeHeader);
-    offset = cur - base;
+    offset = static_cast<uint8_t>(cur - base);
 
 exit:
     return offset;
diff --git a/src/core/mac/mac_frame.cpp b/src/core/mac/mac_frame.cpp
index 74635fbd1..82b11e785 100644
--- a/src/core/mac/mac_frame.cpp
+++ b/src/core/mac/mac_frame.cpp
@@ -1000,7 +1000,7 @@ otError Frame::AppendHeaderIe(HeaderIe *aIeList, uint8_t aIeCount)
         cur += aIeList[i].GetLength();
     }
 
-    SetPsduLength(GetPsduLength() + cur - base);
+    SetPsduLength(GetPsduLength() + static_cast<uint8_t>(cur - base));
 
 exit:
     return error;
diff --git a/src/core/thread/time_sync_service.cpp b/src/core/thread/time_sync_service.cpp
index d9c46b3da..62faf43f6 100644
--- a/src/core/thread/time_sync_service.cpp
+++ b/src/core/thread/time_sync_service.cpp
@@ -52,7 +52,9 @@ TimeSync::TimeSync(Instance &aInstance)
     , mTimeSyncSeq(OT_TIME_SYNC_INVALID_SEQ)
     , mTimeSyncPeriod(OPENTHREAD_CONFIG_TIME_SYNC_PERIOD)
     , mXtalThreshold(OPENTHREAD_CONFIG_TIME_SYNC_XTAL_THRESHOLD)
+#if OPENTHREAD_FTD
     , mLastTimeSyncSent(0)
+#endif
     , mLastTimeSyncReceived(0)
     , mNetworkTimeOffset(0)
 {
@@ -83,7 +85,7 @@ otNetworkTimeStatus TimeSync::GetTime(uint64_t &aNetworkTime) const
         break;
     }
 
-    aNetworkTime = otPlatTimeGet() + mNetworkTimeOffset;
+    aNetworkTime = static_cast<uint64_t>(static_cast<int64_t>(otPlatTimeGet()) + mNetworkTimeOffset);
 
     return networkTimeStatus;
 }
diff --git a/src/core/thread/time_sync_service.hpp b/src/core/thread/time_sync_service.hpp
index 461e0bc41..6d8138297 100644
--- a/src/core/thread/time_sync_service.hpp
+++ b/src/core/thread/time_sync_service.hpp
@@ -144,11 +144,13 @@ private:
      */
     void IncrementTimeSyncSeq(void);
 
-    bool     mTimeSyncRequired;     ///< Indicate whether or not a time synchronization message is required.
-    uint8_t  mTimeSyncSeq;          ///< The time synchronization sequence.
-    uint16_t mTimeSyncPeriod;       ///< The time synchronization period.
-    uint16_t mXtalThreshold;        ///< The XTAL accuracy threshold for a device to become a Router, in PPM.
-    uint32_t mLastTimeSyncSent;     ///< The time when the last time synchronization message was sent.
+    bool     mTimeSyncRequired; ///< Indicate whether or not a time synchronization message is required.
+    uint8_t  mTimeSyncSeq;      ///< The time synchronization sequence.
+    uint16_t mTimeSyncPeriod;   ///< The time synchronization period.
+    uint16_t mXtalThreshold;    ///< The XTAL accuracy threshold for a device to become a Router, in PPM.
+#if OPENTHREAD_FTD
+    uint32_t mLastTimeSyncSent; ///< The time when the last time synchronization message was sent.
+#endif
     uint32_t mLastTimeSyncReceived; ///< The time when the last time synchronization message was received.
     int64_t  mNetworkTimeOffset;    ///< The time offset to the Thread Network time
 };