• Yesterday is history. ^{昨日已成为历史}
• Tomorrow is a mystery.^{明日仍是个谜}
• Today is a gift. ^{今天是一份礼物 (今日为金)}
• That's why it is called "the present".

参考数学中的等量代换：Today => present => gift  ^{通过“present”的一语双关来实现上文要表达的谚语}