```ts
export interface Event {
  name: string;
  timestamp: number;
}

export class EventLog {
  #events: Event[] = [];

  /** Append one event. */
  record(event: Event): void {
    if (!event || typeof event.name !== 'string' || isNaN(event.timestamp)) {
      throw new Error('Invalid event format');
    }
    this.#events.push(event);
  }

  /** Every event at or after the given timestamp, in insertion order. */
  since(timestamp: number): Event[] {
    if (isNaN(timestamp)) {
      throw new Error('Invalid timestamp');
    }
    return this.#events.filter((event) => event.timestamp >= timestamp);
  }
}
```