column A contains dates (UTC formatted) that I wish to format to a particular timezone, although the function is not returning the correct local formatted date.
function LOCALTIME(datetimeString) {
var originalDate = new Date(datetimeString);
return Utilities.formatDate(originalDate, "Australia/Adelaide", "YYYY-MM-DD HH:mm:ss")
}

YYYY-MM-DD HH:mm:ssis modified toyyyy/MM/dd HH:mm:ss, what result will you get? Ref1 And, also "Australia/Adelaide" might beSpreadsheetApp.getActiveSpreadsheet().getSpreadsheetTimeZone(). Ref2...getSpreadsheetTimeZone()Australia/Adelaideis modified toUTC+09:30, what result will you get?2020-05-20T01:46:00.929Zconverted to2020-05-19 16:16:01, should have been2020-05-20 10:16:01that is approx 19 hours behind. It's currently not being converted to the correct timezone of Australia/Adelaide for some reason