--- a/dav/main.c	Sat Oct 07 10:40:21 2017 +0200
+++ b/dav/main.c	Mon Oct 09 14:55:47 2017 +0200
@@ -648,6 +648,13 @@
     time_t t = -1;
     if(update) {
         t = util_parse_lastmodified(update);
+        if (t == 0) {
+            fprintf(stderr,
+                    "Invalid date format. Possible formats are:\n"
+                    "  RFC-1123 - example: Thu, 29 Nov 2012 21:35:35 GMT\n"
+                    "  RFC-3339 - example: 2012-11-29T21:35:35Z\n");
+            return -1;
+        }
     }
     
     int recursive = cmd_getoption(a, "recursive") ? 1 : 0;