From 4d716fa7edde76ed838aefcd8c5945bcfe3ba8f3 Mon Sep 17 00:00:00 2001
From: Stani <spe.stani.be@gmail.com>
Date: Sat, 21 Aug 2021 09:46:43 +0200
Subject: [PATCH] * This closes #1004, new fn: MONTH ref #65

---
 calc.go      | 32 ++++++++++++++++++++++++++++++++
 calc_test.go |  9 +++++++++
 2 files changed, 41 insertions(+)

diff --git a/calc.go b/calc.go
index 9ff0535..9f3e25f 100644
--- a/calc.go
+++ b/calc.go
@@ -420,6 +420,7 @@ type formulaFuncs struct {
 //    MINA
 //    MIRR
 //    MOD
+//    MONTH
 //    MROUND
 //    MULTINOMIAL
 //    MUNIT
@@ -6413,6 +6414,37 @@ func (fn *formulaFuncs) DAY(argsList *list.List) formulaArg {
 	return newNumberFormulaArg(float64(timeFromExcelTime(num.Number, false).Day()))
 }
 
+// MONTH function returns the month of a date represented by a serial number.
+// The month is given as an integer, ranging from 1 (January) to 12
+// (December). The syntax of the function is:
+//
+//    MONTH(serial_number)
+//
+func (fn *formulaFuncs) MONTH(argsList *list.List) formulaArg {
+	if argsList.Len() != 1 {
+		return newErrorFormulaArg(formulaErrorVALUE, "MONTH requires exactly 1 argument")
+	}
+	arg := argsList.Front().Value.(formulaArg)
+	num := arg.ToNumber()
+	if num.Type != ArgNumber {
+		dateString := strings.ToLower(arg.Value())
+		if !isDateOnlyFmt(dateString) {
+			if _, _, _, _, _, err := strToTime(dateString); err.Type == ArgError {
+				return err
+			}
+		}
+		_, month, _, _, err := strToDate(dateString)
+		if err.Type == ArgError {
+			return err
+		}
+		return newNumberFormulaArg(float64(month))
+	}
+	if num.Number < 0 {
+		return newErrorFormulaArg(formulaErrorNUM, "MONTH only accepts positive argument")
+	}
+	return newNumberFormulaArg(float64(timeFromExcelTime(num.Number, false).Month()))
+}
+
 // NOW function returns the current date and time. The function receives no
 // arguments and therefore. The syntax of the function is:
 //
diff --git a/calc_test.go b/calc_test.go
index 8deab93..cea9521 100644
--- a/calc_test.go
+++ b/calc_test.go
@@ -962,6 +962,9 @@ func TestCalcCellValue(t *testing.T) {
 		"=DAY(\"3-February-2008\")":              "3",
 		"=DAY(\"01/25/20\")":                     "25",
 		"=DAY(\"01/25/31\")":                     "25",
+		// MONTH
+		"=MONTH(42171)":           "6",
+		"=MONTH(\"31-May-2015\")": "5",
 		// Text Functions
 		// CHAR
 		"=CHAR(65)": "A",
@@ -1981,6 +1984,12 @@ func TestCalcCellValue(t *testing.T) {
 		"=DAY(\"3-January-9223372036854775808\")":                             "#VALUE!",
 		"=DAY(\"9223372036854775808-January-1900\")":                          "#VALUE!",
 		"=DAY(\"0-January-1900\")":                                            "#VALUE!",
+		// MONTH
+		"=MONTH()":                    "MONTH requires exactly 1 argument",
+		"=MONTH(43891,43101)":         "MONTH requires exactly 1 argument",
+		"=MONTH(-1)":                  "MONTH only accepts positive argument",
+		"=MONTH(\"text\")":            "#VALUE!",
+		"=MONTH(\"January 25, 100\")": "#VALUE!",
 		// NOW
 		"=NOW(A1)": "NOW accepts no arguments",
 		// TODAY