一秒[ichibyou] one second
数秒[suubyou] several seconds
寸秒[sunbyou] a moment
秒針[byoushin] second hand (of clock)
秒速[byousoku] per second

秒読み[byouyomi] countdown
分秒[funbyou] moment
毎秒[maibyou] every second
秒時計[byoudokei] stop watch

正閏[seijun] normal and leap (years)
閏月[uruuzuki] intercalary month
閏年[uruudoshi] leap year

閏秒[uruubyou] leap second
閏日[uruubi] leap day