The painting must date within the period 1832 - 1840, as:

..Klien was promoted to Oberlieutenant in 1832:

.."Klein, Theodor v., Ul. v. [vacant Knesevich Drag. R.], z. Obl. [im R.] ditto."
.. http://books.google.com/books?id=2GBPAAAAcAAJ&pg=PA103

.."Note: In 1834 he is listed as an Oberlieutenant in the (3.) Galizisches Dragoner-Regiment:

..And he was listed as a Zweyte Rittmeister in 1840 in the ](3.) Galizisches Dragoner-Regiment:
.. http://books.google.com/books?id=zYgOZJXuGNoC&pg=PA298

Not our period, but I thought it was nice, eh.

Well, he made it to retirement as an Oberstlieutenant in 1852:

"Klein, Theodor von, Oberstlieutenant; Pensionirung, 26, 127":

"Wien am 25. März 1852.

Der Oberstlieutenant Theodor von Klein, des Meinen Namen führenden 3. Dragoner-Regiments, ist nach dem Superarbitrirungs-Befunde in den zeitlichen Pensionsstand zu übernehmen.

Franz Joseph m. p.

(G.2567, vom 25. März 1852.)":
http://books.google.com/books?id=6r5FAAAAcAAJ&pg=PA127