Crossword Genius

What makes telly so awful about the first of December, according to the Julian calendar (3,5)

Ross

I believe the answer is:

old style

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'julian calendar' is the definition.
(synonyms)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for old style that I've seen before include "Traditional, retro" , "Traditional type" , "how Julius arranged his dates" .)

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