。
The expiration date is March 31, 1997.
Literal
Valid-period [topic-は] 1997-year 3-month 31-day is-[polite-です].
有効期限 ('period of validity, expiration date') is a compound used on coupons, IDs, contracts, and any time-limited documents. 有効 ('valid, effective') is paired with its opposite 無効 ('invalid, void'). Japanese dates read year-month-day, as in this sentence — a consistent big-endian format.