I often hear these mistakes made by German speakers who are learning English.
They say: Last month he has got five new customers.
Remember ‘last month’ describes finished time so the tense is wrong here.
For finished time (yesterday, last week, last month, last year, two days ago etc.) we need to use the simple past tense.
Correction: Last month he got five new customers.
For unfinished time (today, this week, this month, this year etc.) we can use have / has + 3rd form of the verb.
E.g. This month he has got five new customers.
Have fun learning English!
Louise