Short answer: this is a UI limitation, not a Prefect engine limitation. - The Cloud UI’s parameter form currently renders a date picker for

date

but does not provide a proper datetime picker or reliably handle a

date | datetime

union when starting a run. Under the hood, Prefect can validate/parse datetimes just fine; it’s the form control that’s limited. Workarounds you can use today: - Use the JSON editor in the run modal and pass an ISO 8601 string for your datetime. Prefect will parse it to a

datetime

if your parameter is typed accordingly.

{
  "as_of": "2025-09-17T12:30:00Z"
}

- If you want to avoid any UI validation quirks, declare the parameter as

str

and parse inside your flow:

from datetime import datetime
  from prefect import flow

  @flow
  def my_flow(as_of: str):
      dt = datetime.fromisoformat(as_of.replace("Z", "+00:00"))
      # use dt...

- If appropriate, use the flow’s scheduled start time instead of passing a datetime as a parameter (available in

prefect.runtime.flow_run.scheduled_start_time

). If you’re open to it, I can file an issue for better datetime support (and for handling

date | datetime

unions) in the run modal. Also, could you share: - The exact parameter annotation you’re using (e.g.,

date | datetime

), and - Whether you were using the form fields or the JSON editor, plus any error text you saw? That will help us reproduce and get a fix prioritized.