I'm not sure about Excel 11.6 (I don't use Mac), but this is pretty easy in Excel 2010, so I think it will apply.
- Choose your data and insert into an XY chart.
- Select your X-axis and open the format dialog box.
- Manually set the min/max/major/minor values.
- Set min as midnight for day 1 (you'll need to figure Excel's number for that date, e.g. 1/1/2012 00:00:00 = 40909).
- Set max as midnight at end of last day (you'll need to calculate as above), or you can leave it as auto and it'll update as your data does).
- Set major as 1.0, for one day increments (or modify to your preference).
- Set minor as 0.041667 for one hour increments (or modifiy to your preference).
- Format everything else to your taste.
FWIW, with time values, the X-axis will get scrunched pretty fast, so you may not get much discernible detail on exactly where the points fall unless you're very zoomed in. In the sample I've attached, there's only one day shown and the difference between 09:58, 10:00 and 10:10 is difficult to see, so consider how multiple days will impact that.