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APL (Dyalog Extended), 20 19 bytesSBCS



Anonymous tacit prefix function taking [year,month,day] as argument.

2!∘⌊7÷⍨43816-⍨⌂days


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⌂days calculate the number of days since 1899-12-31

43816-⍨ subtract 43816; days since 2019 12 18, i.e 14 days weeks extra

7÷⍨ divide by 7; weeks since 2019 12 18, i.e two weeks extra

⌊ floor; whole weeks since 2019 12 18, i.e two weeks extra

∘

2! count the number of ways to choose 2 out of that many

In other words, if \$d(x)\$ is the number of days since 1899-12-31, then the answer is

$${\Big\lfloor{d(x)-43816\over7}\Big\rfloor\choose2}=
{\Big\lfloor{d(x)-43823\over7}\Big\rfloor-1\choose2}=
\sum_{n=1}^{\big\lfloor\frac{d(x)-43823}7\big\rfloor}n$$