Can I omit 之 in 之后/之前/之内?

Question

In Mandarin, if you want to express "three years after" or "after three years", 之 should be used such that it is 两年之后. This is also true of 之前 and 之内 (e.g. 第二次世界大战之前。, 我三天之内一定做得完。). However, now I wonder if it is possible to omit 之, and write it as 两年后 or 第二次世界大战前. Is this grammatically correct? Or is it wrong but nonetheless used everywhere? (I know these phrases are often used but would like to know if it is correct.)

Answer 1

Yes, you can omit "之" as you want and there is no grammar mistake here

Answer 2

yes, you can omit 之 here, it's Chinese particles.