DateIndex.swift

enum DateIndex {
    // All dates are integers counting from this date
    static let zeroDate = Calendar.current.date(from: .init(year: 1970, month: 1, day: 1))!

    static func fromDate(_ date: Date) -> Int {
        Calendar.current.dateComponents([.day], from: zeroDate, to: date).day!
    }

    static func toDate(_ index: Int) -> Date {
        Calendar.current.date(byAdding: .day, value: index, to: zeroDate)!
    }

    static func today() -> Int {
        fromDate(Date())
    }
}