To get the date on a date or datetime field from a record such as that of startdate or lastmodifieddate, the string output from record.getValue() needs to be converted first to a date using N/format module’s format.parse() method.
require(['N/currentRecord', 'N/format'], function (nCurrentRecord, nFormat) {
var currentRecord = nCurrentRecord.get();
var lastModifiedDateString = currentRecord.getValue({
fieldId: 'startdate',
});
var lastModifiedDateObj = nFormat.parse({
value: lastModifiedDateString,
type: nFormat.Type.DATE
})
console.log(lastModifiedDateObj.getTime());
});