fix year calculation in gmtime

Change-Id: I9a5f29ad73b5620096d38874c70351db9bd90062
diff --git a/firmware/libc/gmtime.c b/firmware/libc/gmtime.c
index 89960ce..b6f75e2 100644
--- a/firmware/libc/gmtime.c
+++ b/firmware/libc/gmtime.c
@@ -77,7 +77,7 @@
             seconds -= YEAR_SECONDS;
         }
     }
-    tm->tm_year = year%100 + 100;
+    tm->tm_year = year - 1900;
 
     /* Month */
     for (i = 0; i < 12; i++)