Timestamped records - including one off the front


I have a table containing timestamped weather data. When I wish to
show the weather for a day I use a named scope:
  named_scope :by_time_range, lambda{ |time_min, time_max|
  {:conditions => ['measured_at >= ? and measured_at <= ?',
time_min.utc, time_max.utc] } }
where time_min and max are midnight to midnight. If there is a sample
at the midnight at the start of the day then all is well, but if there
is not then I have no reference point for the start of the day, so I
would like to include in the find the last sample before midnight if
there is not one at midnight.

I can see how to do this as two finds, one finding the samples where
measured_at > time_min and <= time_max and then concatenating this
with the last sample from a find where measured_at <= time_min, which
will give me the one at midnight or the last one before midnight.

Is there a way I can do this within one find?