Convert mm/dd or mm/dd/yy or mm/dd/yyyy to Excel internal format

I have looked at dozens of articles, but they all talk about converting different forms of mm/dd... etc. to other similar forms. I want to chart blood pressure readings by date and time (different times on different days). If I had the date/time in an internal form, I could chart the day number and time (starting at day 1 at 00:00) for the readings. This will let me chart "real time", i.e., each day (horizontal axis) is the same width, and the time of the reading will be appropriately positioned horizontally within that width, with some days having no entries. I've tried DATE and DATEVALUE with no success. Any help will be appreciated.