dāng nián
in those days
then
in those years
during that time
bù jiǎn dāng nián
(of one's skills, appearance etc) not to have deteriorated a bit
to be as (good, vigorous etc) as ever
hǎo hàn bù tí dāng nián yǒng
a real man doesn't boast about his past achievements (idiom)
zhèng dāng nián
to be in the prime of life