The element is a REQUIRED sub-element of the element. The Atom specification requires that you provide a data and a TIME of when this feed was updated. There is no problem to know the data and time for this element. But you need to pay attention to the format of how the date and time should be specified.

Atom specification requires you to USE the format defined in RFC 3393 for the update element. So you should ONE of the following TWO formats defined in RFC 3339:

yyyy-mm-ddThh:mm:ss.ccZ, where T is a delimiter, and Z indicates this is a UTC time value. For example: 2017-08-12T23:20:50.52Z.

yyyy-mm-ddThh:mm:ss.cczzzzz, where T is a delimiter, and zzzzz is a time zone modifier like -08:00, which is 8 hours behind UTC. For example: 2017-08-12T15:20:50.52-08:00.

The element is a required sub-element of the element. The Atom specification requires that you provide a data and a time of when this feed was updated. There is no problem to know the data and time for this element. But you need to pay attention to the format of how the date and time should be specified.

Atom specification requires you to use the format defined in RFC 3393 for the update element. So you should one of the following two formats defined in RFC 3339:

yyyy-mm-ddThh:mm:ss.ccZ, where T is a delimiter, and Z indicates this is a UTC time value. For example: 2017-08-12T23:20:50.52Z.

yyyy-mm-ddThh:mm:ss.cczzzzz, where T is a delimiter, and zzzzz is a time zone modifier like -08:00, which is 8 hours behind UTC. For example: 2017-08-12T15:20:50.52-08:00.