Present Perfect Simple/Simple Past

Hello,

I have a problem, regarding Present Perfect Simple/Simple Past when it comes to specific/unspecific times.

"In the last year" is an unspecific time so demands Present Perfect Simple

"Last year" is considered an unspecific time so we use Simple Past.

So far so good

But what' s with:

" In the last month"

"Last Month"

And:

"In the morning"

"This morning"

Does the rule apply there too ?

Thank you very much for your help!

Greetings

daxie