org-sort-agenda-notime-is-late

org-sort-agenda-notime-is-late is a variable defined in `org-agenda.el'.
Its value is

t

Documentation:
Non-nil means items without time are considered late.
This is only relevant for sorting. When t, items which have no explicit
time like 15:30 will be considered as 99:01, i.e. later than any items which
do have a time. When nil, the default time is before 0:00. You can use this
option to decide if the schedule for today should come before or after timeless
agenda entries.

You can customize this variable.