Update documentation to use ISO format for dates

The ISO format is more precise and  might be less
confusing for the users.

Signed-off-by: Daniel Zullo <[email protected]>

Author: daniel-zullo-frequenz

src/frequenz/sdk/timeseries/_moving_window.py

@@ -123,7 +123,7 @@ def __init__(  # pylint: disable=too-many-arguments
             resampler_config: The resampler configuration in case resampling is required.
             window_alignment: A datetime object that defines a point in time to which
                 the window is aligned to modulo window size.
-                (default is midnight 01.01.01 UTC)
+                (default is 0001-01-01T00:00:00+00:00)
                 For further information, consult the class level documentation.
 
         Raises: