Joining date on offer letter - how to obtain an offer letter with the correct date?

Hello, I joined the new company on February 13th, and in the offer letter they gave me on February 16th, and before joining date I asked HR to please revised offer letter instead of new offer letter, they gave me written amendment letter with joing date 13th of feb.

No worries about future issues; I asked HR to please revise my offer letter with the correct start date, i.e. February 13th, but they ignored me.

I am currently employed by the same company; please advise me on how to obtain an offer letter with the correct date so that I do not encounter any problems in the future.

Don't need to worry in this scenario as You have revised offer letter. If any encounter fall under any situation you can show them your offer letter with DOJ 13 Feb.

The offer letter together with the amendment makes it amply clear that the joining was on 13th Feb. No need to get worried over it. HR refusing to issue a revised offer is understandable as the purpose is already served.