The figures for 2012-2013 were complicated by a political debt-limit dispute. The nominal national debt typically varies by ± $10 billion from one day to the next, but from June 1 to Sept 30, 2013, the day-to-day variation was 100 times smaller, as the Treasury used "extraordinary measures" (i.e. shuffling money from account to account) to stay just below the limit and avoid default. On Oct. 17, 2013, after the debt-ceiling deal, the official debt figure jumped by $328 billion in one day, presumably "unwinding" these "extraordinary measures", and then returned to its normal ± $10 billion/day variation.
The same thing happened in 2014-2015: from the end of February, 2015 to the end of October, the national debt varied by less than $5 billion, then jumped by $340 billion on Nov. 2, the day Congress increased the debt limit, and by the end of November it had grown by another $335 billion.
These two standoffs are visible as "flat lines" in the following graph, each followed by a jump back to the pre-standoff trend line.
In both cases, the end of the fiscal year fell in the middle of a flat-line, so using the official debt on that day would produce an artificially low number. So I'm interpolating linearly from the debt a month or two later, once the debt ceiling was lifted: I treat 2012-2013 as 13 months long, 2013-2014 as 11 months long, 2014-2015 as 14 months long, and 2015-2016 as 10 months long.