Backtrader uses the floating point datetime representation internally, both for line series (data feeds, indicators and so on) and orders.
This is similar to the way the datetime is used in matplotlib. See https://matplotlib.org/_modules/matplotlib/dates.html for more info.
The float based datetime could be easily converted to python datetime by using num2date() function (see dateintern.py module). Alternatively for data feeds, one may use the data.num2date() to directly get the python datetime for current data feed bar.