Fix call to stat

From Stack Overflow:

GNU stat has -c, BSD stat has -f
stat is not in POSIX at all

kerl

@@ -616,9 +616,8 @@ is_older_than_x_days() {
     # $1: file to check
     # $2: age in days
 
-    eval "$(\stat -s "$1")"
-    # shellcheck disable=SC2154  # .. is referenced but not assigned.
-    if [ $(($(date +%s) - st_mtime)) -gt $(($2 * 24 * 3600)) ]; then
+    kerl_st_mtime=$(\stat -c %X "$1")
+    if [ $(($(date +%s) - kerl_st_mtime)) -gt $(($2 * 24 * 3600)) ]; then
         return 0
     else
         return 1