Added BACKUP_FILE_DATE env variable

- default as `%Y%m%d`

scripts/includes.sh

@@ -287,8 +287,10 @@ function init_env() {
     BACKUP_KEEP_DAYS="${BACKUP_KEEP_DAYS:-"0"}"
 
     # BACKUP_FILE_DATE_FORMAT
+    get_env BACKUP_FILE_DATE
     get_env BACKUP_FILE_DATE_SUFFIX
-    BACKUP_FILE_DATE_FORMAT=$(echo "%Y%m%d${BACKUP_FILE_DATE_SUFFIX}" | sed 's/[^0-9a-zA-Z%_-]//g')
+    BACKUP_FILE_DATE="${BACKUP_FILE_DATE:-"%Y%m%d"}"
+    BACKUP_FILE_DATE_FORMAT=$(echo "${BACKUP_FILE_DATE}${BACKUP_FILE_DATE_SUFFIX}" | sed 's/[^0-9a-zA-Z%_-]//g')
 
     # PING_URL
     get_env PING_URL