Check out this question Google Sheet API V4(Java) append Date in cells, where a similar problem is flagged.
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Google Date Formats
Google Apps Script uses a particular version of JavaScript (ECMA-262 3rd Edition) and as you have discovered can't parse date/times in ISO 8601 format. Below is a modified function I found online:
var dt = new Date(getDateFromIso("2012-08-03T23:00:26-05:00"));
// http://delete.me.uk/2005/03/iso8601.html
function getDateFromIso(string) {
try{
var aDate = new Date();
var regexp = "([0-9]{4})(-([0-9]{2})(-([0-9]{2})" +
"(T([0-9]{2}):([0-9]{2})(:([0-9]{2})(\\.([0-9]+))?)?" +
"(Z|(([-+])([0-9]{2}):([0-9]{2})))?)?)?)?";
var d = string.match(new RegExp(regexp));
var offset = 0;
var date = new Date(d[1], 0, 1);
if (d[3]) { date.setMonth(d[3] - 1); }
if (d[5]) { date.setDate(d[5]); }
if (d[7]) { date.setHours(d[7]); }
if (d[8]) { date.setMinutes(d[8]); }
if (d[10]) { date.setSeconds(d[10]); }
if (d[12]) { date.setMilliseconds(Number("0." + d[12]) * 1000); }
if (d[14]) {
offset = (Number(d[16]) * 60) + Number(d[17]);
offset *= ((d[15] == '-') ? 1 : -1);
}
offset -= date.getTimezoneOffset();
time = (Number(date) + (offset * 60 * 1000));
return aDate.setTime(Number(time));
} catch(e){
return;
}
}
source