--- Time.at(time[, usec])

    time ǻꤷ Time ֥Ȥ֤ޤ
    time  Time ֥ȡ⤷ϵηв
    ɽ魯ưǤ

    ư٤ǤԽʬʾ硢usec ꤷޤ
    time + (usec/1000000) λ֤ޤξ硢
    timeusec ȤǤʤФʤޤ

    줿 Time ֥ȤΥॾȤʤޤ

