In general, we recommend using date/time properties to store dates and time-of-day information. This is because such properties understand how to interpret the values as real date/times. That means you can calculate on them using formulas and you can convert them into different formats.
However, there are some current limitations in Riffyn of date/time properties (which will be addressed eventually). You cannot change the display format of date/time values in the UI, so they will always show in long format like this: 2018-03-26 15:03:12 (which is YYYY-MM-DD HH:MM:SS in 24 hour time). So for the moment, if you cannot tolerate this display format, you may wish to use a character property to store dates or times.
Fortunately, Riffyn is quite smart about data entry, allowing multiple entry formats. But they will still be displayed in the above format.
On the other hand, another advantage of date/time properties is that they enforce more consistency in data capture, and will avoid potential data errors later. And when Riffyn is updated with options for alternative display formats, your historical data will be ready to go.