“First bullpen in the books,” Strasburg tweeted today. “Felt great! Hopefully time will speed up now!”

Since Jan. 31, when Strasburg simply tossed a baseball for the first time since his Sept. 3 elbow reconstruction surgery, Strasburg had been throwing light long toss, making longer and longer throws. Today’s seemingly minor step provides a significant milestone.

Strasburg will continue building arm strength and will eventually throw in minor league rehab assignments. Last year, in his recovery from Tommy John surgery, Jordan Zimmermann threw off a mound for the first time in late March and did not appear in a minor league rehab game until July 3. He made his first major league appearance after surgery August 26.

So if Strasburg takes the same amount of time as Zimmermann from his first time throwing off a mound to his first major league appearance, he would not pitch in the majors this season. But schedules of two pitchers rehabbing from Tommy John can differ, and Nationals officials have often warned of using Zimmermann as a barometer for Strasburg.