diff --git a/src/util.c b/src/util.c
index e7efd02..0eeaede 100644
--- a/src/util.c
+++ b/src/util.c
@@ -1039,7 +1039,7 @@ time_t iso_datetime_read_7(const uint8_t *buf)
     tm.tm_hour = buf[3];
     tm.tm_min = buf[4];
     tm.tm_sec = buf[5];
-    return timegm(&tm) - buf[6] * 60 * 15;
+    return timegm(&tm) - ((int8_t)buf[6]) * 60 * 15;
 }
 
 time_t iso_datetime_read_17(const uint8_t *buf)