The imports of the datetime module are all over the place. We currently have at least the following styles in our lib:

• import datetime
• import datetime as dtm
• from datetime import datetime, timedelta

Let's unify that to import datetime as dtm. I prefer this option since

1. Since there is datetime.datetime, a standalone datetime can be confusing. Using the toplevel module is preferable IMO.
2. dtm is shorter that datetime, plus it make dtm.datetime unambiguous`