Java equivalent for Python's toordinal()?

Question

What is the best way to replicate datetime.datetime.now().toordinal() in java? I tried using epoch but the results are nowhere near.

Answer 1

There isn't any direct way to query that information using the Java Time API but you can calculate it.

public static void main(String[] args) {
    long ordinal = LocalDate.now().toEpochDay() + (146097 * 5L) - (31L * 365L + 7L);

    System.out.println(ordinal);
}

Run today, 19th April 2016, it outputs 736073, which is consistent with Python's output.

From a LocalDate, you can get the Epoch day with toEpochDay(). If you take a look inside the current implementation, you'll find that it actually calculates the total number of days since year 0 and subtracts that with the constant

static final long DAYS_0000_TO_1970 = (DAYS_PER_CYCLE * 5L) - (30L * 365L + 7L);

where DAYS_PER_CYCLE = 146097 is the number of days in a 400 year cycle. That constant isn't public, so we can't reuse it directly.

From the Python documentation of toordinal(), it defines year 1 to have an ordinal of 1. The calculus above by the Java API supposes that it is year 0 instead so we just need to adjust for that.