The way I heard it, the move from Mar to Jan was not for political reasons, but for tax reasons.

Spring makes sense for starting a new year; rebirth and all that. But it's lousy as the start of a tax year: who's to say whether all the new animals in the herds got born last year, and are already taxable, or this year, and are not?

Start your tax year in the middle of winter, however, (like astronomers used to switch days at noon instead of at midnight) and all is good: no livestock is getting born midwinter so the only fuzziness left is did that sheep die this tax year or last? Much more legible.