let date = new Date(2019, 0, 1);
document.write('기준일자 : ' + date + '<br>');

date.setSeconds(date.getSeconds() + 100);
document.write('100초 후 : ' + date + '<br>');

date = new Date(2019, 0, 1);
date.setSeconds(date.getSeconds() - 100);
document.write('100초 전 : ' + date + '<br>');


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