Hebcal's greg package converts between Gregorian/Julian dates and R.D. (Rata Die) day numbers. Dates before September 14, 1752 are interpreted using the Julian calendar.
Example
package main
import (
"fmt"
"time"
"github.com/hebcal/greg"
)
func main() {
rataDie := greg.ToRD(1995, time.December, 17)
fmt.Println(rataDie)
// Output: 728644
}