diff --git a/src/lib/spinel/radio_spinel_impl.hpp b/src/lib/spinel/radio_spinel_impl.hpp
index d8f136ccb..612443bb7 100644
--- a/src/lib/spinel/radio_spinel_impl.hpp
+++ b/src/lib/spinel/radio_spinel_impl.hpp
@@ -2133,7 +2133,7 @@ void RadioSpinel<InterfaceType, ProcessContextType>::CalcRcpTimeOffset(void)
 
     VerifyOrExit(error == OT_ERROR_NONE, mRadioTimeRecalcStart = localRxTimestamp);
 
-    mRadioTimeOffset      = remoteTimestamp - ((localRxTimestamp / 2) + (localTxTimestamp / 2));
+    mRadioTimeOffset      = static_cast<int64_t>(remoteTimestamp - ((localRxTimestamp / 2) + (localTxTimestamp / 2)));
     mIsTimeSynced         = true;
     mRadioTimeRecalcStart = localRxTimestamp + RCP_TIME_OFFSET_CHECK_INTERVAL;