Request for Pomodoro clock review and help

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This is not the proper use of this.setState.

Why do not you link your element with id “timer-label” directly to the state?

<div id='timer-label'>{this.state.label}</div>

Another thing: do not allow the countdown reach negative values. Your code says:

(Line #142) if (starttime < 0)

This must replace by

if (starttime == 0) {
   this.setState({starttime: 'Break'})
   statetements...
}