A long-time employee of a New York school system could lose his job for clocking out at the same place at the same time while carrying a work-issued cell phone with GPS that showed him at home during the hours he claims he was at work.

A 21-year employee of the school system could lose his job after officials accused him of repeatedly leaving early - and stunned the worker with data it got by tracking his movements with a city-issued cellphone, The Post has learned.

In a precedent-setting case, administrative trial judge Tynia Richard recommended the firing of John Halpin, a veteran supervisor of carpenters, for cutting out before the end of his shift on as many as 83 occasions between March 2 and Aug. 9, 2006.

The evidence against Halpin, whose base pay is $300 a day, included time cards that suspiciously appeared stamped on the same machine, even though his duties placed him in different locations each day.

But there was a clincher: data gathered through the GPS system on Halpin’s cellphone, which he accepted in 2005 without being told it might be used to trace his every move.

On March 8, for example, supervisors determined that Halpin was last in Manhattan at 1:31 p.m. and was home in Levittown, L.I., at 2:40 p.m. On March 29, Halpin was found at home at 2:38 p.m.

The earliest he was caught in Levittown was 1:40 p.m. on June 22.

But his shift wasn’t supposed to end until 3:30 p.m.

Some workers refused the free-phone offer, saying they preferred to use their own cells.

Looks like he should have had his own cell phone too.