На сентяорь 21 сентября, мы свидетельствовали убийство.
It’s not the ending, which is OK, but the translation that is highly questionable.
If what you mean by On September 21, we witnessed a murder is On September 21, we became witnesses of a murder being committed right in front of our eyes the translation would be 21го cентября мы стали/были свидетелями убийства

21 сентября, мы (за)свидетельствовали убийство. basically means that we signed the papers whereby we confirmed the act of murder having been performed.