Where can I find the full specification of a timestamp?

[Ruijie Yu via "General discussions about Org-mode." <emacs-orgmode@gnu.org>]

Hello,

In the process of trying to fix all non-default-locale-related test
failures, I ran into a weird-looking timestamp somewhere around
testing/lisp/test-org.el:5969, in test `test-org/deadline':

<2021-07-20 -1d>

I couldn't find relevant information from the info page on the first try
(by having a quick scan in (info "(org) Dates and Times")).  The
previous comment says it is something called "warning period", which
then hints me at (info "(org) Deadlines and Scheduling").

My question is, what would the full specification of a timestamp be?
Or, phrased differently, what else am I missing, and where can I find
documentation for these optional fields?

Thanks.

-- 
Best,


RY

[Please note that this mail might go to spam due to some
misconfiguration in my mail server -- still investigating.]