I’m guessing you didn’t mean HH:mm:ss for a date (in the list all placeholders):
I haven’t found documentation on that. I tried <$Date:YYYY/MM/DD>, but DD was replaced with the day number (273 instead of 30).
Also, a suggestion: Please allow text in that window to be searched for and also copied and pasted.
Doubt it is a typo as the date comes from the format used in System Preferences > Language & Region > Advanced > Dates > Short. The time reference creates a timestamp instead of a date.
For example, this:
DATE <$date> AT TIME <$date:HH:mm:ss>
DATE 30/09/2018 AT TIME 20:11:04
And if you are talking about the List of All Placeholders, you should be able to copy and paste from that window.
You can use the Pages file attached of the List of All Placeholders to search.
List of All Placeholders.zip (346 KB)