Til

[Maybo]

Thanks for letting me know, till the 18th.

I heard someone say that.
What is that mean.

 

[Tdol]

It's a strange sentence. Without more context, I think they may have meant to say Please let me know by the 18th, setting the 18th as the limit.

 

[emsr2d2]

Thanks for letting me know, till the 18th.

I heard someone say that. What [STRIKE]is[/STRIKE] does [STRIKE]that[/STRIKE] it mean?

I agree with Tdol. The sentence, as it stands, does not work.

Please let me know by the 18th. (That's what you would say before the person lets you know whatever it is.)
Thanks for letting me know. (That's what you would say after the person lets you know whatever it is.)
Thanks for letting me know by the 18th. (You could say that after the person has let you know whatever it is if you want to make it clear that you are grateful that they managed to let you know by the previously agreed date - the 18th.)