date-fns の differenceInDays を気軽に使うとハマる件

気をつけましょう…

ハマり点その1

console.log( differenceInDays(new Date(1951, 4, 7), new Date(1951, 4, 8)) );
console.log( differenceInDays(new Date(1951, 4, 6), new Date(1951, 4, 7)) );
console.log( differenceInDays(new Date(1951, 4, 5), new Date(1951, 4, 6)) );

↓

-1
0   👈
-1

これは1951年5月6日が夏時間開始日で23時間しかなかったから、
というわけでUTCで計算すれば

console.log( differenceInDays(Date.UTC(1951, 4, 7), Date.UTC(1951, 4, 8)) );
console.log( differenceInDays(Date.UTC(1951, 4, 6), Date.UTC(1951, 4, 7)) );
console.log( differenceInDays(Date.UTC(1951, 4, 5), Date.UTC(1951, 4, 6)) );

↓

-1
-1
-1

OK。

ハマり点その2

console.log( differenceInDays(Date.UTC(1951, 8, 10), Date.UTC(1958, 6, 1)), (Date.UTC(1951, 8, 10) - Date.UTC(1958 ,6, 1)) / 86400000 );
console.log( differenceInDays(Date.UTC(1951, 8,  9), Date.UTC(1958, 6, 1)), (Date.UTC(1951, 8,  9) - Date.UTC(1975, 6,1 )) / 86400000 );
console.log( differenceInDays(Date.UTC(1951, 8,  8), Date.UTC(1958, 6, 1)), (Date.UTC(1951, 8,  8) - Date.UTC(1975, 6,1 )) / 86400000 );
console.log( differenceInDays(Date.UTC(1951, 8,  7), Date.UTC(1958, 6, 1)), (Date.UTC(1951, 8,  7) - Date.UTC(1975, 6,1 )) / 86400000 );
console.log( differenceInDays(Date.UTC(1951, 8,  6), Date.UTC(1958, 6, 1)), (Date.UTC(1951, 8,  6) - Date.UTC(1975, 6,1 )) / 86400000 );

↓

-2486 -2486
-2487 -2487
-2487 -2488  👈
-2488 -2489  👈
-2489 -2490  👈

！？
（ちなみにこの1951年9月8日は平和条約が締結された日ですが、関係あるのだろうか？）