I've seen both used to mean "X ago", e.g. "六日前と" and "3ヶ月前に".
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question
particles particle-に particle-と
What is the difference between 前と and 前に?
> What is the difference between 前と and 前に?  
>  I've seen both used to been "X ago", e.g. "六日前と" and "3ヶ月前に".

Those you've seen must be both correct, but に is the one to say 'X ago'.

と is used for a different reason such as 六日前と三か月前に, or 六日前と言っていました.

**に** indicates the location of something including **time** or owner of
things or ideas or values or etc.  
**と** **connects** nouns or noun-like words, or is used to describe something
by _indicating the **content** of a quotation or thought or idea,_ and the
form of the content doesn't matter whether it's a word or phrase or clause;
六日前 **と** いう話, 六日前の話 **と** いうことです, 「六日前に彼と会った」と言いました, etc.
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2017-07-31T00:40:32.377
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answer
Case examples I guess are as follows.

> -（Ａ）彼｛かれ｝が私｛わたし｝を訪｛たず｝ねて来｛き｝たのは **六日前と** 思｛おも｝う。 _I think it was six days
> ago when he visited me_.  
>  -（Ｂ）彼は **六日前に** 私を訪ねて来た。 _He visited me six days ago_.

(A) makes sense, but it'll become better if you edit it as 彼が私を訪ねて来たのは六日前
**だ** と思う。or 彼が私を訪ねて来たのは六日前 **である** と思う。 More precisely, it is 彼が私を訪ねて来た
**日｛ひ｝** は、六日前であると思う。

**と** in this phrase is a particle used for quoting (with speech, thoughts,
etc.); quoting particle​. With this particle 六日前 becomes the quoted object of
the verb 思う.

On the other hand, phrase (B) is a very common usage of に, which is a particle
indicating time.
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answer