Jason Terry doesn’t need to look at a calendar. At the age of 35 with 14 NBA seasons under his belt, the vibrations of the nearing playoffs are running through his body. He knows April begins Monday, and he knows that 4-for-18 shooting on 3-pointers over the six games heading into last night is not going to cut it for a guy being counted on to come off the bench and loosen opponents’ defenses. On a night when he became the fourth player in NBA history to make 1,900 3-pointers (joining Ray Allen, Reggie Miller and Jason Kidd), Terry proved he hasn’t lost his sense of timing. “He understands that,” said Paul Pierce, who managed to secure a triple-double (20 points, 10 rebounds, 10 assists) while Terry was going for 24 points off the pine in last night’s 118-107 win over Atlanta. “He’s a veteran. He knows that the playoffs are right around the corner, so he’s really picked up his production. He’s picked up his shooting. He’s being a lot more aggressive for us, and we’re going to need that bench scoring in the playoffs. He’s just a veteran who’s been around the block a few times, and he understands the moments and that time is winding down getting ready for the playoffs.” Terry has shrugged off some recent struggles by noting that he will be there when the bell rings for the bigger affairs. But his own game was caught in the rut with that of the Celts lately.