todo-legacy-date-time-regexp

todo-legacy-date-time-regexp is a variable defined in `todo-mode.el'.
Its value is

"\\(?1:[0-9]\\{4\\}\\)-\\(?2:[0-9]\\{2\\}\\)-\\(?3:[0-9]\\{2\\}\\) \\(?4:[0-9]\\{2\\}:[0-9]\\{2\\}\\)"

Documentation:
Regexp matching legacy todo-mode.el item date-time strings.
In order for `todo-convert-legacy-files' to correctly convert
this string to the current Todo mode format, the regexp must
contain four explicitly numbered groups (see `(elisp) Regexp
Backslash'), where group 1 matches a string for the year, group 2
a string for the month, group 3 a string for the day and group 4
a string for the time. The default value converts date-time
strings built using the default value of
`todo-time-string-format' from todo-mode.el.

You can customize this variable.