Some already can (like PipeOpImputeConstant). I don't know which of the others support it right now.
At the very least, we should add it to PipeOpImpute's private$.train_nullmodel() where I initially noted it missing.

TODO:

• Check for all imputation PipeOps (incl. the abstract base class) whether they currently support POSIXct features or if they theoretically could. In that case implement it.
• Set the feature_types argument to super$initialize() to the correct values.
• Modify tests to test whether the respective PipeOp behaves as expected for features of this types.