How does Date promote to DateTime in EK9?

← Date, Time, and Duration · Ref: Q552

Date automatically promotes to DateTime via the #^ operator. DateTime can also be decomposed back into its Date and Time parts.

DATE TO DATETIME PROMOTION

The #^ promote operator converts Date to DateTime (midnight UTC):

  birthday <- 1971-02-01
  birthdayDT <- #^ birthday
  birthdayDT is 1971-02-01T00:00:00Z

Automatic promotion also occurs when assigning Date to a DateTime variable:

  dtValue as DateTime: birthday
  dtValue is 1971-02-01T00:00:00Z

Or via the DateTime(Date) constructor:

  explicit <- DateTime(birthday)

DATETIME TO DATE AND TIME EXTRACTION

The date() method extracts the Date portion:

  myDate <- myDateTime.date()

The time() method extracts the Time portion:

  myTime <- myDateTime.time()

Alternatively, use extraction operators:

  #< extracts the Date (lower/coarser part)
  #> extracts the Time (upper/finer part)
  datePart <- #< myDateTime
  timePart <- #> myDateTime

MILLISECOND TO DURATION

Similar promotion: Millisecond promotes to Duration via #^:

  ms <- 5000ms
  dur <- #^ ms
  dur is PT5S

See Q25 for the promote operator in general. See Q31 for Date and Time basics. See Q92 for DateTime with timezones. See Q554 for extracting DateTime components.

Example

defines module qa.date.promote.datetime

  defines program
    DatePromoteDateTimeDemo()
      stdout <- Stdout()

      birthday <- 1971-02-01

      // Explicit promote operator
      promoted <- #^ birthday
      stdout.println(`Promoted: ${promoted}`)

      // Assignment promotion
      dtValue as DateTime: birthday
      stdout.println(`Assigned: ${dtValue}`)

      // Constructor promotion
      explicit <- DateTime(birthday)
      stdout.println(`Constructor: ${explicit}`)

      // All three are equivalent
      require promoted == dtValue
      require dtValue == explicit

      // DateTime decomposition
      meeting <- 2024-06-15T10:30:00Z

      // Method extraction
      datePart <- meeting.date()
      timePart <- meeting.time()
      stdout.println(`Date part: ${datePart}`)
      stdout.println(`Time part: ${timePart}`)

      // Operator extraction
      dateExtract <- #< meeting
      timeExtract <- #> meeting
      stdout.println(`#< extract: ${dateExtract}`)
      stdout.println(`#> extract: ${timeExtract}`)

      require datePart == dateExtract
      require timePart == timeExtract

      // Millisecond to Duration promotion
      ms <- 5000ms
      dur <- #^ ms
      stdout.println(`5000ms promoted: ${dur}`)

      // Round-trip: Date -> DateTime -> Date
      original <- 2024-03-15
      asDT as DateTime: original
      backToDate <- asDT.date()
      require backToDate == original
      stdout.println(`Round-trip: ${original} -> ${asDT} -> ${backToDate}`)

Common mistakes

E50060 — DateTime has no toDate() or toTime() methods. Use date() and time() to extract the Date and Time components. See ek9 -h E50060 for details.

Incorrect:

datePart <- meeting.toDate()
      timePart <- meeting.toTime()

Correct:

datePart <- meeting.date()
      timePart <- meeting.time()

Other ways to ask this

How do I convert a Date to DateTime?
What does the #^ promote operator do with Date?
How do I extract Date and Time from DateTime?

Coming from another language?

Java: LocalDate.atStartOfDay(ZoneOffset.UTC) to convert Date to DateTime. LocalDateTime.toLocalDate() and toLocalTime() for extraction. Python: datetime.combine(date, time.min) for promotion. Go: no separate Date type. Rust: NaiveDate with_hms() methods. EK9: #^ promotes Date to DateTime automatically, date() and time() extract parts, #< and #> extraction operators.

Keywords: extract, date, time, constructor, timezone, promote, date(), time(), convert, datetime, decompose, duration