first base.syncState() works but one immediately after does not

[yamini-krishnamurthy]

This is how my state looks:

this.state = {
      fullness: 6,
      count: 1,
      lastFed: this.props.startDate,
}

This is what my componentDidMount (kind of) looks like:

componentDidMount() {
  this.fullnessRef = base.syncState(`users/${this.props.uid}/fullness`, {
    context: this,
    state: 'fullness',
  })

  this.lastFedRef = base.syncState(`users/${this.props.uid}/lastFed`, {
    context: this,
    state: 'lastFed',
  })

  if(this.props.newUser === false) {
    const hoursPassed = new Date() - this.state.lastFed
    const reduceFullnessBy = Math.floor(hoursPassed / (1000 * 60))

    const fullness =  this.state.fullness -
      reduceFullnessBy

    this.setState({ fullness })
  }
}

I see users/uid/fullness appear on firebase, but users/uid/lastFed (a Date) will not appear. In fact, when I use React dev tools, this.state.lastFed becomes undefined all of a sudden. Not entirely sure what I'm doing wrong. Does syncState not work with Date objects?