"He talks in March."
Translation:Er spricht im März.
First of all "März" is masculine, so the accusative would be "in den März". But you don't need an accusative here, but a dative, since a static position is given,not a movement. The latter would be like saying "into March" in English.
Maybe you got the rule wrong: motion = accusative, no motion = dative.
It is not considered a motion. A motion is an action of movement from A to B. But a speaker usually stands still on his/her place. And even if he/she didn't, it is not the speakinh which causes the motion.
"im März" is static, because the point in time remains statich in March (doesn't move). "in den März" would be like saying "into march", which would be a motion (in time).