If you import the data from your text file into a column of date/time data type the date element would default to 30 December 1899, which is 'day zero' in Access's implementation of the date/time data type. As you are importing the data for exactly one day at a time, then you could set the database up so that the relevant date is first entered into a text box in an unbound dialogue form, and after each import session execute an update query to add the date to each value where the date element is 30 December 1899, e.g.
PARAMETERS Forms!frmDateDlg!txtDate DATETIME;
UPDATE WaterLog
SET HourOfDay = HourOfDay + Forms!frmDateDlg!txtDate
WHERE DATEVALUE(HourOfDay) = #1899-12-30#;