Variable: org-datetree-base-level
org-datetree-base-level is a variable defined in org-datetree.el.gz.
Value
1
Documentation
The level at which years should be placed in the date tree.
This is normally one, but if the buffer has an entry with a DATE_TREE (or WEEK_TREE for ISO week entries) property (any value), the date tree will become a subtree under that entry, so the base level will be properly adjusted.
Source Code
;; Defined in /usr/src/emacs/lisp/org/org-datetree.el.gz
(defvar org-datetree-base-level 1
"The level at which years should be placed in the date tree.
This is normally one, but if the buffer has an entry with a
DATE_TREE (or WEEK_TREE for ISO week entries) property (any
value), the date tree will become a subtree under that entry, so
the base level will be properly adjusted.")