Understanding Percentage Comparisons in Data Analysis

When I use percentage comparison, I’m not sure what it’s based on. For example, it shows 34% growth for the last 30 days, but when I check the last 30 days, it should actually be a drop. I don’t know what date range the percentage is calculated on. For example, in MTD (Sep 1–14), the comparison is with the same period last month (Aug 1–14).