[NeoN]

возможно, вы хотели что-то вроде этого:

var x;
var someDate=new Date(1600,1,9); //произвольная дата
if (someDate>=new Date(1582,9,15)&&someDate<=new Date(1700,02,11)) x = 10;
else if (someDate>=new Date(1700,02,12)&&someDate<=new Date(1800,02,12)) x = 11;
else if (someDate>=new Date(1800,02,13)&&someDate<=new Date(1900,02,13)) x = 12;
else if (someDate>=new Date(1900,02,14)&&someDate<=new Date(2100,02,14)) x = 13;
else if (someDate>=new Date(2100,02,15)&&someDate<=new Date(2200,02,15)) x = 14;
else if (someDate>=new Date(2200,02,16)&&someDate<=new Date(2300,02,16)) x = 15;