This is the 7263rd most frequent Korean word.
지난달부터
"Since last month."
The word "지난달부터" indicates the starting time (last month) from which the action (learning tennis) began.
나는 지난달부터 테니스를 배우기 시작했어요.
I started learning tennis since last month.
Here, "지난달부터" expresses the time period since last month when the action (selling the new product) became ongoing.
우리 회사는 지난달부터 신제품을 판매하고 있습니다.
Our company has been selling the new product since last month.
In this sentence, "지난달부터" marks the commencement time (last month) for a continuous activity (working overtime every day).