diff --git a/hw/rtc/m48t59.c b/hw/rtc/m48t59.c
index ec3e56e84f..2e2c849985 100644
--- a/hw/rtc/m48t59.c
+++ b/hw/rtc/m48t59.c
@@ -133,7 +133,7 @@ static void alarm_cb (void *opaque)
 
 static void set_alarm(M48t59State *NVRAM)
 {
-    int diff;
+    int64_t diff;
     if (NVRAM->alrm_timer != NULL) {
         timer_del(NVRAM->alrm_timer);
         diff = qemu_timedate_diff(&NVRAM->alarm) - NVRAM->time_offset;