Add helper function to calculate days in between, and add function to…

… return date time in ISO 8601 format.

nepali-date-picker/src/commonMain/kotlin/dev/shivathapaa/nepalidatepickerkmp/calendar_model/DateConverters.kt

@@ -263,16 +263,7 @@ internal object DateConverters {
         return totalNepDaysCount
     }
 
-    // Three overload functions for month details
-    fun getNepaliMonth(
-        nepaliYear: Int, nepaliMonth: Int
-    ): NepaliMonthCalendar {
-        // Calculate and return the month details
-        return calculateNepaliMonthDetails(
-            nepaliYear = nepaliYear, nepaliMonth = nepaliMonth
-        )
-    }
-
+    // Two overload functions for month details
     fun getNepaliMonth(
         nepaliYear: Int, nepaliMonth: Int, addedMonthsCount: Int
     ): NepaliMonthCalendar {
@@ -291,6 +282,32 @@ internal object DateConverters {
         )
     }
 
+    /**
+     * Calculates the total number of days between two [SimpleDate] objects in the Nepali calendar.
+     *
+     * @param startDate The starting date.
+     * @param endDate The ending date.
+     * @return The number of days between the two dates. Returns -1 if either date is invalid,
+     *         and throws exception if the year is not found in [daysInMonthMap].
+     */
+    fun nepaliDaysInBetween(startDate: SimpleDate, endDate: SimpleDate): Int {
+        if (startDate.year > endDate.year) {
+            return -nepaliDaysInBetween(endDate, startDate)
+        }
+
+        if (!isNepaliCalendarInConversionRange(startDate.year, startDate.month, startDate.dayOfMonth)
+            || !isNepaliCalendarInConversionRange(endDate.year, endDate.month, endDate.dayOfMonth)
+        ) {
+            throw IllegalArgumentException("Out of Range: Nepali start year ${startDate.year} or end year " +
+                    "${startDate.year} is out of range to compare. Check range value from NepaliDatePickerDefaults.")
+        }
+
+        val startOffset = calculateDayOffset(minNepaliYear, startDate.year, startDate.month) + startDate.dayOfMonth
+        val endOffset = calculateDayOffset(minNepaliYear, endDate.year, endDate.month) + endDate.dayOfMonth
+
+        return endOffset - startOffset
+    }
+
     private fun getCustomCalendarUsingDayMonthYear(
         year: Int,
         month: Int,

nepali-date-picker/src/commonMain/kotlin/dev/shivathapaa/nepalidatepickerkmp/calendar_model/NepaliCalendarModel.kt

@@ -28,6 +28,7 @@ import dev.shivathapaa.nepalidatepickerkmp.data.SimpleTime
 import kotlinx.datetime.Clock
 import kotlinx.datetime.LocalDateTime
 import kotlinx.datetime.TimeZone
+import kotlinx.datetime.toInstant
 import kotlinx.datetime.toLocalDateTime
 
 @Immutable
@@ -83,16 +84,8 @@ internal class NepaliCalendarModel(val locale: NepaliDateLocale = NepaliDateLoca
         return DateConverters.getTotalDaysInNepaliMonth(year, month)
     }
 
-    fun calculateFirstAndLastDayOfNepaliMonth(
-        nepaliYear: Int, nepaliMonth: Int
-    ): NepaliMonthCalendar {
-        return DateConverters.calculateNepaliMonthDetails(nepaliYear, nepaliMonth)
-    }
-
     fun getNepaliMonth(nepaliYear: Int, nepaliMonth: Int): NepaliMonthCalendar {
-        return DateConverters.getNepaliMonth(
-            nepaliYear = nepaliYear, nepaliMonth = nepaliMonth
-        )
+        return DateConverters.calculateNepaliMonthDetails(nepaliYear, nepaliMonth)
     }
 
     fun getNepaliMonth(simpleNepaliDate: SimpleDate): CustomCalendar {
@@ -121,6 +114,40 @@ internal class NepaliCalendarModel(val locale: NepaliDateLocale = NepaliDateLoca
         )
     }
 
+    fun nepaliDaysInBetween(startDate: SimpleDate, endDate: SimpleDate): Int {
+        return DateConverters.nepaliDaysInBetween(startDate, endDate)
+    }
+
+    fun formatEnglishDateToIsoFormat(englishDate: SimpleDate, time: SimpleTime): String {
+        val localDateTime = LocalDateTime(
+            englishDate.year,
+            englishDate.month,
+            englishDate.dayOfMonth,
+            time.hour,
+            time.minute,
+            time.second,
+            time.nanosecond
+        )
+
+        return localDateTime.toInstant(timeZone).toString()
+    }
+
+    fun formatNepaliDateToIsoFormat(nepaliDate: SimpleDate, time: SimpleTime): String {
+        val convertedEnglishDate = convertToEnglishDate(nepaliDate.year, nepaliDate.month, nepaliDate.dayOfMonth)
+
+        val localDateTime = LocalDateTime(
+            convertedEnglishDate.year,
+            convertedEnglishDate.month,
+            convertedEnglishDate.dayOfMonth,
+            time.hour,
+            time.minute,
+            time.second,
+            time.nanosecond
+        )
+
+        return localDateTime.toInstant(timeZone).toString()
+    }
+
     /**
      * Formats a Nepali date based on the specified user preferences.
      *

nepali-date-picker/src/commonMain/kotlin/dev/shivathapaa/nepalidatepickerkmp/calendar_model/NepaliDateConverter.kt

@@ -26,6 +26,8 @@ import dev.shivathapaa.nepalidatepickerkmp.data.NepaliMonthCalendar
 import dev.shivathapaa.nepalidatepickerkmp.data.SimpleDate
 import dev.shivathapaa.nepalidatepickerkmp.data.SimpleTime
 import dev.shivathapaa.nepalidatepickerkmp.data.englishMonths
+import kotlinx.datetime.LocalDate
+import kotlinx.datetime.daysUntil
 
 @Immutable
 object NepaliDateConverter {
@@ -108,7 +110,7 @@ object NepaliDateConverter {
     fun getNepaliMonthCalendar(
         nepaliYear: Int, nepaliMonth: Int
     ): NepaliMonthCalendar {
-        return calendarModel.calculateFirstAndLastDayOfNepaliMonth(nepaliYear, nepaliMonth)
+        return calendarModel.getNepaliMonth(nepaliYear, nepaliMonth)
     }
 
     /**
@@ -296,6 +298,81 @@ object NepaliDateConverter {
         )
     }
 
+    /**
+     * Calculates the total number of days between two [SimpleDate] objects in the Nepali calendar.
+     *
+     * The end date is not added. You can add 1 to its returned value to include the end date too.
+     *
+     * @param startDate The starting date.
+     * @param endDate The ending date.
+     * @return The number of days between the two dates. Throws exception if the year is not in
+     * range [NepaliDatePickerDefaults.NepaliYearRange], or returns -1 if either date is invalid.
+     */
+    fun getNepaliDaysInBetween(startDate: SimpleDate, endDate: SimpleDate): Int {
+        return calendarModel.nepaliDaysInBetween(startDate, endDate)
+    }
+
+    /**
+     * Calculates the total number of days between two [SimpleDate] objects in the English calendar.
+     *
+     * The end date is not added. You can add 1 to its returned value to include the end date too.
+     *
+     * @param startDate The starting date.
+     * @param endDate The ending date.
+     * @return The number of days between the two dates.
+     */
+    fun getEnglishDaysInBetween(startDate: SimpleDate, endDate: SimpleDate): Int {
+        val startLocalDate = LocalDate(startDate.year, startDate.month, startDate.dayOfMonth)
+        val endLocalDate = LocalDate(endDate.year, endDate.month, endDate.dayOfMonth)
+
+        return startLocalDate.daysUntil(endLocalDate)
+    }
+
+    /**
+     * Converts an English (Gregorian) date and time to ISO 8601 UTC format.
+     *
+     * @param englishDate The date in the Gregorian calendar (English date) as a [SimpleDate].
+     * @param time The time of day as a [SimpleTime], default is the current time.
+     *
+     * @return A string in ISO 8601 format representing the UTC date and time.
+     *         The result is in the format `YYYY-MM-DDTHH:mm:ssZ` (Zulu time).
+     *
+     * Example:
+     * ```
+     * val englishDate = SimpleDate(2024, 9, 9)
+     * val time = SimpleTime(14, 30, 15, 0)
+     * val isoFormat = NepaliDateConverter.formatEnglishDateToIsoFormat(englishDate, time)
+     * println(isoFormat)  // Outputs: "2024-09-09T09:00:15Z"
+     * ```
+     */
+    fun formatEnglishDateToIsoFormat(englishDate: SimpleDate, time: SimpleTime = currentTime): String {
+        return calendarModel.formatEnglishDateToIsoFormat(englishDate, time)
+    }
+
+    /**
+     * Converts a Nepali (Bikram Sambat) date and time to ISO 8601 UTC format.
+     *
+     * The Nepali date (Bikram Sambat) is first converted to the equivalent Gregorian (English) date,
+     * and then the time is appended to produce a complete timestamp in UTC.
+     *
+     * @param nepaliDate The date in the Nepali Bikram Sambat calendar as a [SimpleDate].
+     * @param time The time of day as a [SimpleTime], default is the current time.
+     *
+     * @return A string in ISO 8601 format representing the UTC date and time.
+     *         The result is in the format `YYYY-MM-DDTHH:mm:ssZ` (Zulu time).
+     *
+     * Example:
+     * ```
+     * val nepaliDate = SimpleDate(2081, 5, 24)  // Bikram Sambat date
+     * val time = SimpleTime(14, 30, 15, 0)
+     * val isoFormat = NepaliDateConverter.formatNepaliDateToIsoFormat(nepaliDate, time)
+     * println(isoFormat)  // Outputs: "2024-09-09T09:00:15Z"
+     * ```
+     */
+    fun formatNepaliDateToIsoFormat(nepaliDate: SimpleDate, time: SimpleTime = currentTime): String {
+        return calendarModel.formatNepaliDateToIsoFormat(nepaliDate, time)
+    }
+
     /**
      * @param dayOfWeek takes value between 1 to 7.
      * @param format gives name of day either in short or medium or full name. i.e. Sunday, Mon, T, आईतबार, आईत, आ, etc.