Hi I Have Irregular Periods. Im A 18 Years Old

Hi
I have irregular periods. Im a 18 years old girl and Single. I wenn to my doctor yesterday. She suggest me to take Duphaston 10mg each a Day from 16 ZT - 26 ZT, But the Problem is that i didnt understand what she meant with ( 16 ZT - 26 ZT ) and i dont know
When should I Start to take the Medicine After , during or before my periods and how long , because I dont know when i will get my next periods .

Please answer me.