If you're running a bank that was established in 1958, for example, you might pick 50 as the baseline. Any year-dates greater than 50 are assumed to belong to the 20th century, while any year-dates less than that are assumed to belong to the 21st century. 58 automatically means 1958, while 48 means 2048.
譬如某家銀行現存的最早開戶記錄是一九五八年，於是以五○年為基準年份，超過五○的，像是五八，就視為一九五八年；小於五○的，像是四八，就視為二○四八年。