Convert date components (per RFC3339) to seconds since 1970-01-01

toTimestamp.swift

func toTimestamp(year: Int, month: Int, day: Int, hour: Int, minute: Int, seconds: Int, nanoseconds: Int, offsetInSeconds: Int) -> Double {
    var year = year
    year -= month <= 2 ? 1 : 0
    let era = (year >= 0 ? year : year - 399) / 400
    let yoe = year - era * 400
    let doy = (153*(month + (month > 2 ? -3 : 9)) + 2)/5 + day - 1
    let doe = yoe * 365 + yoe/4 - yoe/100 + doy
    let dayCounts = era * 146097 + doe - 719468
    let seconds = dayCounts * 86400 + hour * 3600 + minute * 60 + seconds - offsetInSeconds
    return Double(seconds) + Double(nanoseconds) / 1_000_000_000
}