Example of date interval math using Julian day numbers

tuple-dates.js

const fromIso = isoDate => {
    const intDiv = (l,r) => ~~(l/r)
    const [y, m, d] = isoDate.split('-').map(s => parseInt(s, 10))
    return intDiv(1461 * (y + 4800 + intDiv(m - 14, 12)), 4)
        + intDiv(367 * (m - 2 - 12 * intDiv(m - 14, 12)), 12)
        - intDiv(3 * intDiv(y + 4900 + intDiv(m - 14, 12), 100), 4)
        + d - 32075
}

const fromIsoInterval = isoInterval =>
    isoInterval.split('/').map(fromIso)

const intervalOverlap = (a, b) => !(a[0] > b[1] || a[1] < b[0])

// Assumes overlaps is already checked, will give garbage if no overlap
const intervalIntersection = (a, b) =>
    a[0] <= b[0]
        ? a[1] >= b[1]
            ? b
            : [b[0], a[1]]
        : a[1] >= b[1]
            ? [a[0], b[1]]
            : a

// same assumption as intersection
const intervalDifference = (a, b) => { 
    const ivls = []
    const [f,s] = a[0] <= b[0] ? [a,b] : [b,a]
    const [l,m] = a[1] >= b[1] ? [a,b] : [b,a]
    if (f[0] !== s[0])
        ivls.push([f[0], s[0] - 1])
    if (l[1] !== m[1])
        ivls.push([m[1] + 1, l[1]])
    return ivls
}

module.exports = {
    fromIso,
    fromIsoInterval,
    intervalOverlap,
    intervalIntersection,
    intervalDifference
}