fixes bug introduced in commit d8020972
fixes bug introduced in commit d8020972

To simplify the conditional logic, I introduced the `conv_dtime_at`
variable but never set it to `dtime_at` when it is a datetime object.
As a result passing a datetime object would always result in getting
the current time back.

djaodjin/djaodjin-deployutils,djaodjin/djaodjin-deployutils,djaodjin/djaodjin-deployutils