hourglass.ts

class Hourglass {
  private ticker: ([number] | [number, number])[] = [];

  public tic() {
    const d = this.ticker.slice(-1)[0];

    if (!this.ticker.length || d[1]) {
      this.ticker.push([Date.now()]);
    }
  }

  public tok() {
    const d = this.ticker.slice(-1)[0];

    if (this.ticker.length && !d[1]) {
      d[1] = Date.now();
    }
  }

  public sum() {
    return (this.ticker.map((v, idx) =>
      idx === v[1] ? v : [v[0], Date.now()]
    ) as [number, number][]).reduce((acc, v) => acc + (v[1] - v[0]), 0);
  }
}

hourglass2.ts

class Hourglass2 {
  private start = 0;
  private total = 0;

  public tic() {
    this.start = Date.now();
  }

  public tok() {
    if (this.start) {
      this.total += Date.now() - this.start;
      this.start = 0;
    }
  }

  public sum() {
    this.tok();
    return this.total;
  }
}