Updating state on props change in React Form

I am having trouble with a React form and managing the state properly. I have a time input field in a form (in a modal). The initial value is set as a state variable in getInitialState, and is passed in from a parent component. This in itself works fine.

The problem comes when I want to update the default start_time value through the parent component. The update itself happens in the parent component through setState start_time: new_time. However in my form, the default start_time value never changes, since it is only defined once in getInitialState.

I have tried to use componentWillUpdate to force a change in state through setState start_time: next_props.start_time, which did actually work, but it gave me Uncaught RangeError: Maximum call stack size exceeded errors.

So my question is, what's the correct way of updating state in this case? Am I thinking about this wrong somehow?

Current Code:

@ModalBody = React.createClass

  getInitialState: ->

    start_time: @props.start_time.format("HH:mm")



  #works but takes long and causes:

  #"Uncaught RangeError: Maximum call stack size exceeded"

  componentWillUpdate: (next_props, next_state) ->

    @setState(start_time: next_props.start_time.format("HH:mm"))



  fieldChanged: (fieldName, event) ->

    stateUpdate = {}

    stateUpdate[fieldName] = event.target.value

    @setState(stateUpdate)



  render: ->

    React.DOM.div

      className: "modal-body"

      React.DOM.form null,

        React.createElement FormLabelInputField,

          type: "time"

          id: "start_time"

          label_name: "Start Time"

          value: @state.start_time

          onChange: @fieldChanged.bind(null, "start_time")



@FormLabelInputField = React.createClass

  render: ->

    React.DOM.div

      className: "form-group"

      React.DOM.label

        htmlFor: @props.id

        @props.label_name + ": "

      React.DOM.input

        className: "form-control"

        type: @props.type

        id: @props.id

        value: @props.value

        onChange: @props.onChange