bk(): Remove the separators between the units of time

In PR/60 we discussed the use of the local timezone instead of UTC. While doing
so we figured out that the current implementation is broken on VFAT.

This solution removes the delimiters between the units of time which is fully
IS0 8601 compliant and should work well on all filesystems.

etc/zsh/zshrc

@@ -3268,7 +3268,7 @@ fi
 #f5# Backup \kbd{file_or_folder {\rm to} file_or_folder\_timestamp}
 function bk () {
     emulate -L zsh
-    local current_date=$(date -u "+%Y-%m-%dT%H:%M:%SZ")
+    local current_date=$(date -u "+%Y%m%dT%H%M%SZ")
     local clean keep move verbose result all to_bk
     setopt extended_glob
     keep=1
@@ -3325,14 +3325,14 @@ return 0;;
     elif (( clean > 0 )); then
         if (( $# > 0 )); then
             for to_bk in "$@"; do
-                rm $verbose -rf "${to_bk%/}"_[0-9](#c4,)-(0[0-9]|1[0-2])-([0-2][0-9]|3[0-1])T([0-1][0-9]|2[0-3])(:[0-5][0-9])(#c2)Z
+                rm $verbose -rf "${to_bk%/}"_[0-9](#c8)T([0-1][0-9]|2[0-3])([0-5][0-9])(#c2)Z
                 (( result += $? ))
             done
         else
             if (( all > 0 )); then
-                rm $verbose -rf *_[0-9](#c4,)-(0[0-9]|1[0-2])-([0-2][0-9]|3[0-1])T([0-1][0-9]|2[0-3])(:[0-5][0-9])(#c2)Z(D)
+                rm $verbose -rf *_[0-9](#c8)T([0-1][0-9]|2[0-3])([0-5][0-9])(#c2)Z(D)
             else
-                rm $verbose -rf *_[0-9](#c4,)-(0[0-9]|1[0-2])-([0-2][0-9]|3[0-1])T([0-1][0-9]|2[0-3])(:[0-5][0-9])(#c2)Z
+                rm $verbose -rf *_[0-9](#c8)T([0-1][0-9]|2[0-3])([0-5][0-9])(#c2)Z
             fi
             (( result += $? ))
         fi