Small Basic Pseudo Clock Object

Clock_DateToJD.sb

Sub Clock_DateToJD
  ' Clock | convert date to Julian Day
  ' param date - date formatted as YYYY/MM/DD
  ' return jd - Julian Day [day]
  s1 = Text.GetIndexOf(date, "/")
  year = Text.ConvertToUpperCase(Text.GetSubText(date, 1, s1 - 1))
  bc = Text.GetIndexOf(year, "BC")
  If 0 < bc Then
    year = Text.GetSubText(year, 1, bc - 1) + Text.GetSubTextToEnd(year, bc + 2)
    year = -(year - 1)
  EndIf
  s2 = Text.GetIndexOf(Text.GetSubTextToEnd(date, s1 + 1), "/") + s1
  month = Text.GetSubText(date, s1 + 1, s2 - s1 - 1)
  day = Text.GetSubTextToEnd(date, s2 + 1)
  jc = "False"  ' Julian calendar
  If year < 1582 Then
    jc = "True"
  ElseIf year = 1582 Then
    If month < 10 Then
      jc = "True"
    ElseIf month = 10 Then
      If day <= 4 Then
        jc = "True"
      ElseIf 4 < day And day < 15 Then
        TextWindow.WriteLine("ERROR:Invalid date " + date)
      EndIf
    EndIf
  EndIf
  If jc Then  ' Julian calendar
    y4801BC = year + 4800
    d4801BC = Math.Ceiling(y4801BC * 365.25)
    If Math.Remainder(y4801BC, 4) = 0 Then
      dom[2] = 29
    Else
      dom[2] = 28
    EndIf
    For mm = 1 To month - 1
      d4801BC = d4801BC + dom[mm]
    EndFor
    d4801BC = d4801BC + day - 1
    jd = d4801BC - 32142.5
  Else        ' Gregorian calendar
    y1200 = year - 1200
    nol = Math.Ceiling(y1200 / 4) - Math.Ceiling(y1200 / 100) + Math.Ceiling(y1200 / 400)  ' number of leap year
    d1200 = y1200 * 365 + nol
    If Math.Remainder(y1200, 4) = 0 And Math.Remainder(y1200, 100) <> 0 Or Math.Remainder(y1200, 400) = 0 Then
      dom[2] = 29
    Else
      dom[2] = 28
    EndIf
    For mm = 1 To month - 1
      d1200 = d1200 + dom[mm]
    EndFor
    d1200 = d1200 + day - 1
    jd = d1200 + 2159350.5
  EndIf
EndSub

Clock_Init.sb

Sub Clock_Init
  ' Clock | initialize days of months
  dom = "1=31;2=28;3=31;4=30;5=31;6=30;7=31;8=31;9=30;10=31;11=30;12=31;"
EndSub

Clock_JDNow

Sub Clock_JDNow
  ' Clock | get Julian Day for now
  ' return jd - Julian Day [day]
  jd = Clock.ElapsedMilliseconds / 1000 / 3600 / 24 + 2415020.5
EndSub

Clock_JDToDate.sb

Sub Clock_JDToDate
  ' Clock | convert Julian Day to date
  ' param jd - Julian Day [day] ≧ -32142.5 (-4801 BC/01/01)
  ' return date - date formatted as YYYY/MM/DD
  If jd < 2299160.5 Then  ' Julian calendar
    d4801BC = Math.Floor(jd + 0.5) - 0.5 + 32142.5
    year = (-4800) + Math.Floor(d4801BC / 365.25)
    If Math.Remainder(year, 4) = 0 Then
      dom[2] = 29
    Else
      dom[2] = 28
    EndIf
    day = Math.Floor(Math.Remainder(d4801BC, 365.25))
    If year < 1 Then
      year = (Math.Abs(year) + 1) + " BC"
    EndIF
  Else                    ' Gregorian calendar
    d1200 = Math.Floor(jd + 0.5) - 0.5 - 2159350.5
    y1200 = Math.Floor(d1200 / 365.2425)
    year = 1200 + y1200
    If Math.Remainder(year, 4) = 0 And Math.Remainder(year, 100) <> 0 Or Math.Remainder(year, 400) = 0 Then
      dom[2] = 29
    Else
      dom[2] = 28
    EndIf
    nol = Math.Ceiling(y1200 / 4) - Math.Ceiling(y1200 / 100) + Math.Ceiling(y1200 / 400)  ' number of leap year
    day = d1200 - nol
    day = Math.Floor(Math.Remainder(d1200 - nol, 365))
  EndIf
  For mm = 1 To 12
    If day < dom[mm] Then
      month = mm
      mm = 12  ' exit For
    Else
      day = day - dom[mm]
    EndIf
  EndFor
  If month < 10 Then
    month = Text.Append(0, month * 1)
  EndIf
  day = day + 1
  If day < 10 Then
    day = Text.Append(0, day * 1)
  EndIf
  date = year + "/" + month + "/" + day
EndSub

Added comments for revision 2.