Correct ISO 8601 date-time regular expression to include "Z"

[litera]

Z is for UTC or Zulu time.
ISO 8601 standard should be checked for exact format that should be used.

Standard defines combined date and time as:

<date>T<time>

where date is most commonly represented by YYYYMMDD and time is commonly represented by HH:MM:SS. Additionally some other parts are appended to designate time as local or in relation to UTC.

<time>Z
<time>±hh:mm
<time>±hhmm
<time>±hh

All of the above should be covered by regular expression.

Decision whether UTC times (Zulu) should be converted to local times is part of further discussion and user needs.