Use core with fixed date formatting

Signed-off-by: Raphaël Barrois <raphael.barrois@polytechnique.org>

diff --git a/core b/core
index bf0ebb5..ef815e7 160000 (submodule)
--- a/core
+++ b/core
@@ -1 +1 @@
-Subproject commit bf0ebb51d77a061ae970a1303c6b1a6f614ef86b
+Subproject commit ef815e754cee390b7503d848cf753f0cef328d6b