Why does the Deep Space Atomic Clock have such a short mission?

The Deep Space Atomic Clock has a short mission of only two years. How it can be useful for space operations if its operating for such a short time?

Additionally I would like to ask if there are or were more clock-only satellites in orbit.

• @uhoh: sorry, I keep typing links with TiddlyWiki syntax and then botching the change to Markdown, so there was an extra ]. Fixed now! Also added a note on possible extension as you say. – tfb Jul 22 '20 at 8:21