// You don't need the constructor if you use initializers on the fields:
class SW {
    private startTime: number = 0;
    // −−−−−−−−−−−−−−−−−−−−−−^^^^
    private endTime: number = 0;
    // −−−−−−−−−−−−−−−−−−−−^^^^

    start() {
        this.startTime = Date.now();
    }

    stop() {
        this.endTime = Date.now();
    }

    getDuration() {
        const seconds = (this.endTime - this.startTime) / 1000;
    }
}

// In fact, if you do that, you don't even need to write the type explicitly
// because TypeScript will infer `number` from the initializer
class SW {
    private startTime = 0;
    // −−−−−−−−−−−−−−^^^^
    private endTime = 0;
    // −−−−−−−−−−−−^^^^

    start() {
        this.startTime = Date.now();
    }

    stop() {
        this.endTime = Date.now();
    }

    getDuration() {
        const seconds = (this.endTime - this.startTime) / 1000;
    }
}