I wanted to share this since it took me a lot of searching and testing different codes to find ones that actually worked. The algorithms were coded in 'C++'. I converted them into B4J. Just put them in the same module.

B4X:

```
Sub MiladiIsLeap(miladiYear As Int) As Int
If(((miladiYear Mod 100) <> 0 And (miladiYear Mod 4) = 0) Or ((miladiYear Mod 100) = 0 And (miladiYear Mod 400) = 0)) Then
Return 1
Else
Return 0
End If
End Sub
'Persian to Gregorian conversion. Takes the Persian year/month/day as parameters.
Sub ShamsiToMiladi(y As Int, m As Int, d As Int) As String
{
Dim sumshamsi() As Int = Array As Int ( 31, 62, 93, 124, 155, 186, 216, 246, 276, 306, 336, 365 )
Dim miladidays() As Int = Array As Int( 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 )
Dim yy, mm, dd,daycount As Int
daycount = d
If m > 1 Then daycount = daycount+ sumshamsi(m - 2)
yy = y + 621
daycount = daycount + 79
If MiladiIsLeap(yy) = 1 Then
If daycount > 366 Then
daycount = daycount - 366
yy = yy + 1
End If
Else If daycount > 365 Then
daycount = daycount - 365
yy = yy + 1
End If
If MiladiIsLeap(yy) = 1 Then miladidays(1) = 29
mm=0
Do While (daycount > miladidays(mm))
daycount = daycount - miladidays(mm)
mm = mm + 1
Loop
Dim iYear As Int = yy
Dim iMonth As Int = (mm + 1)
Dim iDay As Int = daycount
Return iYear & "/" & NumberFormat(iMonth, 2, 0) & "/" & NumberFormat(iDay, 2, 0)
End Sub
'Convert Gregorian date to Hijri Shamsi (Persian) date
Sub MiladiToShamsi(Year As Int, Month As Int, Day As Int) As String
Dim count_days(12) As Int
count_days = Array As Int(31,28,31,30,31,30,31,31,30,31,30,31)
Dim i As Int
Dim day_year As Int
Dim newMonth, newYear, newDay As Int
day_year = 0
For i = 1 To Month - 1
day_year = day_year + count_days(i-1)
Next
day_year = day_year + Day
If IsLeapYear(Year) And Month > 2 Then day_year = day_year + 1
If day_year <= 79 Then
If (Year - 1) Mod 4 = 0 Then
day_year = day_year + 11
Else
day_year = day_year + 10
End If
newYear = Year - 622
If day_year Mod 30 = 0 Then
newMonth = (day_year / 30) + 9
newDay = 30
Else
newMonth = (day_year / 30) + 10
newDay = day_year Mod 30
End If
Else
newYear = Year - 621
day_year = day_year - 79
If day_year <= 186 Then
If day_year Mod 31 = 0 Then
newMonth = day_year / 31
newDay = 31
Else
newMonth = (day_year / 31) + 1
newDay = day_year Mod 31
End If
Else
day_year = day_year - 186
If day_year Mod 30 = 0 Then
newMonth = (day_year / 30) + 6
newDay = 30
Else
newMonth = (day_year / 30) + 7
newDay = day_year Mod 30
End If
End If
End If
'Return the result
Return newYear & "/" & newMonth & "/" & newDay
End Sub
Sub IsLeapYear(year As Int) As Boolean
If year Mod 400 = 0 Then Return True
If year Mod 100 = 0 Then Return False
If year Mod 4 = 0 Then Return True
Return False
End Sub
```

Among all the things I found in different websites, these ones seem to work flawlessly. I hope it proves useful to someone.