Add and describe DateTime (timestamp)

[henkbirkholz]

First we have to understand what we want to do here. There are two prominent options:

1. A composite value of calendar values (the counting of days and years) & time values (the counting of seconds) as defined in, for example:
   • https://www.rfc-editor.org/rfc/rfc3339#section-5.6 "Internet Date/Time Format" and
   • ISO 8601-1:2019 "Date and time — Representations for information interchange — Part 1: Basic rules"
2. A time value as defined in, for example:
   • IEEE Std 1003.1-2017 "IEEE Standard for Information Technology--Portable Operating System Interface (POSIX(TM)) Base Specifications" Section 4.16 "Seconds Since the Epoch"

Do we want both? Do we need both? Do we need something else?

[joergwa]

use time ontology https://www.w3.org/TR/owl-time/