Nifty/Banknifty Options weekly expiry symbol names

What is the naming convention for the weekly expiry symbols for Nifty & Banknifty? There seems to be two ways,

For 12Dec19 expiry, the symbol looks like NIFTY19D1212100CE .

But for 20Jan10 expiry, the symbol looks like NIFTY2010212100CE .

How do we decide between "D" and "1"?

Is there a webpage where the naming convention is specified?

Appreciate a quick answer.

Ramakrishnan S
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