What kind of data are you storing in the datetme? Is the lead actually going to be able to choose from any date @ any time?
There isn't currently a way to allow a lead to enter a date and a time in the same field.
Do you need them to be able to choose any date and any time? Or is it a list of events on specific days at specific times?
If that's the case then you could use a string field and make each of the Date/Time options in a picklist.