```ts
export function mergeIntervals(intervals: number[][]): number[][] {
  if (!intervals || intervals.length === 0) return [];

  // Sort intervals by start time
  intervals.sort((a, b) => a[0] - b[0]);

  const merged: number[][] = [intervals[0]];

  for (let i = 1; i < intervals.length; i++) {
    const currentStart = intervals[i][0];
    const currentEnd = intervals[i][1];
    const lastMergedEnd = merged[merged.length - 1][1];

    // If the current interval starts before or at the end of the last merged interval
    if (currentStart <= lastMergedEnd) {
      // Merge them by updating the end of the last merged interval
      intervals[i] = [lastMergedEnd, Math.max(currentEnd, lastMergedEnd)];
    }
  }

  return merged;
}
```