BeakerX supports them by including the already existing contributed extension. The View -> Cell Toolbar -> Initialization Cell menu item makes the UI visible. It also adds a calculator button icon to the toolbar to run the init cells.
Initialization cells are not run unless a notebook is trusted. Click on the trust button to enable this feature on a given notebook.
import time
print(time.strftime("%Y-%m-%d %H:%M"))
2017-07-16 22:37