~て + duration + になる (elapsed time)

The pattern [verb]-て + [duration] + になる expresses 'X amount of time has passed since Y happened.' Literally 'having done Y, [time] has come into being.' Compact way to express elapsed time relative to a triggering event: 結婚して30年になる ('it's been 30 years since [we] married'), 日本に来て5年になる ('it's been 5 years since I came to Japan').