ISO 8601 format date code example

Example 1: iso-8601

echo date('c'); //2021-02-03T21:42:51-08:00

echo date(DATE_ISO8601); //2021-02-03T21:42:51-0800

$datetime = new DateTime();
echo $datetime->format(DateTime::ATOM); //2021-02-03T21:42:51-08:00

Example 2: ISO 8601

YYYY-MM-DD'T'hh:mm:ss'Z'

Z is the ofset from the UTC timezone 
it can be replaced with "(+/-)hh:mm"
if ofset is not specified, character 'Z' is in place instead

2021-01-01T01:01:01+01:00
2021-02-02T02:02:42.069-02:00
2021-03-03T03:03:03.3Z
20210404T040404Z