The mode function is defined for both duration and datetime arrays, so I would have expected it to work. Can you post a small sample of data (say 10-20 elements in T) along with what you expected mode to return when called with that sample so we can take a look?
Also please update the Release section on the right side of this page (below the Tags section) in case the behavior of datetime, duration, and/or mode have changed in a later release than the one you're using.
