Scala date parsing with optional time and optional time zone

DateParsing.scala

import java.time.format.{ DateTimeFormatterBuilder}
import java.time.format.DateTimeFormatter.{ISO_LOCAL_DATE, ISO_LOCAL_TIME}
import java.time.{LocalDate, LocalDateTime, OffsetDateTime, ZoneOffset}
import scala.util.{Success, Try}

object DateParsing extends App {

  private val formatter = new DateTimeFormatterBuilder()
    .parseCaseInsensitive
    .append(ISO_LOCAL_DATE)
    .optionalStart()
    .appendLiteral('T')
    .append(ISO_LOCAL_TIME)
    .optionalStart
    .appendOffsetId
    .toFormatter

  def dateFromISOEither(s: String): Either[String, OffsetDateTime] =
    Try(formatter.parseBest(s, OffsetDateTime.from(_), LocalDateTime.from(_), LocalDate.from(_))) match {
      case Success(a: OffsetDateTime) => Right(a)
      case Success(a: LocalDateTime) => Right(a.atZone(ZoneOffset.UTC).toOffsetDateTime)
      case Success(a: LocalDate) => Right(a.atStartOfDay(ZoneOffset.UTC).toOffsetDateTime)
      case _ => Left(s"Error parsing $s to OffsetDateTime")
    }

  List(
    "2021-01-19",
    "2021-01-19T10:04:35",
    "2021-01-19T10:04:35Z",
    "2021-01-19T10:04:35+01:00",
  ).map(dateFromISOEither).foreach(println)

}