Number + 期 / Modifier + 期
The noun 期 (qī) means "period," "phase," or "term" and is used to denote a span of time. It commonly appears in compound words like 时期 (era), 学期 (semester), and 日期 (date), and can also function as a measure word for scheduled issues or installments.
这个学期我选了五门课。
Zhège xuéqī wǒ xuǎn le wǔ mén kè.
I chose five courses this semester.
那段时期他过得很辛苦。
Nà duàn shíqī tā guò de hěn xīnkǔ.
He had a very hard time during that period.
请在表格上填写出生日期。
Qǐng zài biǎogé shàng tiánxiě chūshēng rìqī.
Please fill in your date of birth on the form.