handle exporting and importing events repeating every Xth day

app/src/main/kotlin/com/simplemobiletools/calendar/helpers/IcsExporter.kt

@@ -111,6 +111,12 @@ class IcsExporter {
         } else if (event.repeatInterval.isXMonthlyRepetition()) {
             if (event.repeatRule == REPEAT_MONTH_LAST_DAY) {
                 ";$BYMONTHDAY=-1"
+            } else if (event.repeatRule == REPEAT_MONTH_EVERY_XTH_DAY) {
+                val start = Formatter.getDateTimeFromTS(event.startTS)
+                val dayOfMonth = start.dayOfMonth
+                val order = (dayOfMonth - 1) / 7 + 1
+                val day = getDayLetters(start.dayOfWeek)
+                ";$BYDAY=$order$day"
             } else {
                 ""
             }
@@ -138,6 +144,18 @@ class IcsExporter {
         return result.trimEnd(',')
     }
 
+    private fun getDayLetters(dayOfWeek: Int): String {
+        return when (dayOfWeek) {
+            1 -> MO
+            2 -> TU
+            3 -> WE
+            4 -> TH
+            5 -> FR
+            6 -> SA
+            else -> SU
+        }
+    }
+
     private fun fillReminders(event: Event, out: BufferedWriter) {
         checkReminder(event.reminder1Minutes, out)
         checkReminder(event.reminder2Minutes, out)

app/src/main/kotlin/com/simplemobiletools/calendar/helpers/IcsImporter.kt

@@ -3,6 +3,8 @@ package com.simplemobiletools.calendar.helpers
 import android.content.Context
 import com.simplemobiletools.calendar.R
 import com.simplemobiletools.calendar.extensions.dbHelper
+import com.simplemobiletools.calendar.extensions.isXMonthlyRepetition
+import com.simplemobiletools.calendar.extensions.isXWeeklyRepetition
 import com.simplemobiletools.calendar.extensions.seconds
 import com.simplemobiletools.calendar.helpers.IcsImporter.ImportResult.*
 import com.simplemobiletools.calendar.models.Event
@@ -212,7 +214,11 @@ class IcsImporter {
             } else if (key == INTERVAL) {
                 frequencySeconds *= value.toInt()
             } else if (key == BYDAY) {
-                handleRepeatRule(value)
+                if (frequencySeconds.isXWeeklyRepetition()) {
+                    handleRepeatRule(value)
+                } else if (frequencySeconds.isXMonthlyRepetition()) {
+                    curRepeatRule = REPEAT_MONTH_EVERY_XTH_DAY
+                }
             } else if (key == BYMONTHDAY && value.toInt() == -1) {
                 curRepeatRule = REPEAT_MONTH_LAST_DAY
             }