"In the last 3 months" vs "in the past 3 months"

What's the difference between in the last 3 months and in the past 3 months if there is any?

Solution 1:

A lot of people now accept that last and past can be used interchangeably in certain contexts:

• These last few months have been difficult.
• These past few months have been difficult.

However, traditional grammarians claim you should never use last when you mean past since last is final as opposed to just gone by.

These last few months have been difficult.

Old-schoolers would say that unless you are dying, you probably have more months ahead of you. Therefore, it would be more appropriate to say, “These past few months have been difficult.”

The bottom line: These days it doesn’t matter if you use last or past, but if you wanted to be hardcore about it, you could use the more appropriate choice based on context.

http://www.angelaelson.com/2013/01/13/grammar-tip-passed-vs-past-vs-last/

Solution 2:

I would say that there is no distinction. If one wanted to say 1 August---1 October, one could say 'the last three calendar months' or 'the past three calendar months', I suppose.

Solution 3:

Today is Oct. 13, 2010.

It can be argued that in the last 3 months would be intuitively understood as the time frame from 8/13/2010 to 10/12/2010, while in the past three months would mean July, August, and September. Some (see comments) see it exactly the other way around. Therefore there seems to be no difference between the two, but if you want to be precise, you need to add something like

in the last three calendar months

or use another formulation like

during/within the last 90 days

Solution 4:

“in the last 3 months” - I guess this term should be used with past simple. The time period is already gone. E.g. now is february and if I say "in the last 3 months", I mean "in november, december and january".

The term “in the past 3 months" shows the period which is not finished yet. So there should be used the present perfect.