org-extend-today-until

org-extend-today-until is a variable defined in `org.el'.
Its value is

0

Documentation:
The hour when your day really ends. Must be an integer.
This has influence for the following applications:
- When switching the agenda to "today". It it is still earlier than
the time given here, the day recognized as TODAY is actually yesterday.
- When a date is read from the user and it is still before the time given
here, the current date and time will be assumed to be yesterday, 23:59.
Also, timestamps inserted in capture templates follow this rule.

IMPORTANT: This is a feature whose implementation is and likely will
remain incomplete. Really, it is only here because past midnight seems to
be the favorite working time of John Wiegley :-)

You can customize this variable.