How can I add a timestamp at any arbitrary time point? For instance, if I want to add a timestamp for 19:00 while I'm at 20:00, how can I do that? ​

In a text block: type +timestamp and hit enter.

Then option click on the timestamp...

...to change the time using the date picker.

Alternative formulation of the question:

How can I create and edit a link to a specific time? ​