Date time validation

Try this

a!localVariables(
  local!selectedTime: now(),
  a!dateTimeField(
    label: "Select Time (max 2h from now)",
    value: local!selectedTime,
    saveInto: local!selectedTime,
    validations: (
      if(
        local!selectedTime > addhours(now(), 2),
        "Select time within 2 hours from current time",
        {}
      )
    )
  )
)

In addition to this, if you're pre-setting the value i.e. in "local!selectedTime", and it's the save target as seen here, assuming it's in a task where the value is expected to be saved somewhere, the "Submit" button (preferably, among various options) will need to make sure to save the value contained in "local!selectedTime" into the Rule Input value (and/or whatever CDT or Record Data is being passed out of the form upon submission), otherwise the user's selected value will just get lost upon submission.

In addition to this, if you're pre-setting the value i.e. in "local!selectedTime", and it's the save target as seen here, assuming it's in a task where the value is expected to be saved somewhere, the "Submit" button (preferably, among various options) will need to make sure to save the value contained in "local!selectedTime" into the Rule Input value (and/or whatever CDT or Record Data is being passed out of the form upon submission), otherwise the user's selected value will just get lost upon submission.