diff --git a/spreadsheet/formula/fnfinance.go b/spreadsheet/formula/fnfinance.go
index b9c41735..5f3d4c4c 100644
--- a/spreadsheet/formula/fnfinance.go
+++ b/spreadsheet/formula/fnfinance.go
@@ -384,6 +384,9 @@ func Accrintm(args []Result) Result {
 	if errResult.Type == ResultTypeError {
 		return errResult
 	}
+	if issueDate >= settlementDate {
+		return MakeErrorResultType(ErrorTypeNum, "Issue date should be earlier than settlement date")
+	}
 	if args[2].Type != ResultTypeNumber {
 		return MakeErrorResult("ACCRINTM requires rate to be number argument")
 	}
diff --git a/spreadsheet/formula/fnmathtrig.go b/spreadsheet/formula/fnmathtrig.go
index 9c147c45..d9c09d7d 100644
--- a/spreadsheet/formula/fnmathtrig.go
+++ b/spreadsheet/formula/fnmathtrig.go
@@ -1376,8 +1376,8 @@ const (
 // Round is an implementation of the Excel ROUND function that rounds a number
 // to a specified number of digits.
 func round(args []Result, mode rmode) Result {
-	if len(args) == 0 {
-		return MakeErrorResult("ROUND() requires at least one numeric arguments")
+	if len(args) != 2 {
+		return MakeErrorResult("ROUND() requires two numeric arguments")
 	}
 	// number to round
 	number := args[0].AsNumber()
@@ -1385,14 +1385,11 @@ func round(args []Result, mode rmode) Result {
 		return MakeErrorResult("first argument to ROUND() must be a number")
 	}
 
-	digits := float64(0)
-	if len(args) > 1 {
-		digitArg := args[1].AsNumber()
-		if digitArg.Type != ResultTypeNumber {
-			return MakeErrorResult("second argument to ROUND() must be a number")
-		}
-		digits = digitArg.ValueNumber
+	digitArg := args[1].AsNumber()
+	if digitArg.Type != ResultTypeNumber {
+		return MakeErrorResult("second argument to ROUND() must be a number")
 	}
+	digits := digitArg.ValueNumber
 
 	v := number.ValueNumber
 
diff --git a/spreadsheet/formula/fntext.go b/spreadsheet/formula/fntext.go
index 899dd1c4..04621320 100644
--- a/spreadsheet/formula/fntext.go
+++ b/spreadsheet/formula/fntext.go
@@ -33,6 +33,7 @@ func init() {
 	RegisterFunction("LEN", Len)
 	RegisterFunction("LENB", Len)
 	RegisterFunction("LOWER", Lower)
+	RegisterFunction("MID", Mid)
 	RegisterFunction("PROPER", Proper)
 	RegisterFunction("REPLACE", Replace)
 	RegisterFunction("REPT", Rept)
@@ -40,6 +41,7 @@ func init() {
 	RegisterFunction("RIGHTB", Right)
 	RegisterFunction("SEARCH", Search)
 	RegisterFunctionComplex("SEARCHB", Searchb)
+	RegisterFunction("SUBSTITUTE", Substitute)
 	RegisterFunction("T", T)
 	RegisterFunction("TEXT", Text)
 	RegisterFunction("TEXTJOIN", TextJoin)
@@ -380,6 +382,43 @@ func lower(arg Result) Result {
 	}
 }
 
+// Mid is an implementation of the Excel MID function that returns a copy
+// of the string with each word capitalized.
+func Mid(args []Result) Result {
+	if len(args) != 3 {
+		return MakeErrorResult("MID requires three arguments")
+	}
+	if args[0].Type != ResultTypeString {
+		return MakeErrorResult("MID requires text to be a string")
+	}
+	text := args[0].ValueString
+	if args[1].Type != ResultTypeNumber {
+		return MakeErrorResult("MID requires start_num to be a number")
+	}
+	startNum := int(args[1].ValueNumber)
+	if startNum < 1 {
+		return MakeErrorResult("MID requires start_num to be more than 0")
+	}
+	if args[2].Type != ResultTypeNumber {
+		return MakeErrorResult("MID requires num_chars to be a number")
+	}
+	numChars := int(args[2].ValueNumber)
+	if numChars < 0 {
+		return MakeErrorResult("MID requires num_chars to be non negative")
+	}
+	l := len(text)
+	if startNum > l {
+		return MakeStringResult("")
+	}
+	startNum--
+	endNum := startNum + numChars
+	if endNum > l + 1 {
+		return MakeStringResult(text[startNum:])
+	} else {
+		return MakeStringResult(text[startNum:endNum])
+	}
+}
+
 // Proper is an implementation of the Excel PROPER function that returns a copy
 // of the string with each word capitalized.
 func Proper(args []Result) Result {
@@ -534,6 +573,67 @@ func Searchb(ctx Context, ev Evaluator, args []Result) Result {
 	return MakeErrorResultType(ErrorTypeValue, "Not found")
 }
 
+// Substitute is an implementation of the Excel SUBSTITUTE function.
+func Substitute(args []Result) Result {
+	argsNum := len(args)
+	if argsNum != 3 && argsNum != 4 {
+		return MakeErrorResult("SUBSTITUTE requires three or four arguments")
+	}
+	text, errResult := getString(args[0], "SUBSTITUTE", "text")
+	if errResult.Type == ResultTypeError {
+		return errResult
+	}
+	oldText, errResult := getString(args[1], "SUBSTITUTE", "old text")
+	if errResult.Type == ResultTypeError {
+		return errResult
+	}
+	newText, errResult := getString(args[2], "SUBSTITUTE", "new text")
+	if errResult.Type == ResultTypeError {
+		return errResult
+	}
+	instanceNum := 0
+	if argsNum == 3 {
+		return MakeStringResult(strings.Replace(text, oldText, newText, -1))
+	} else {
+		instanceNumF, errResult := getNumber(args[3], "SUBSTITUTE", "instance_num")
+		if errResult.Type == ResultTypeError {
+			return errResult
+		}
+		instanceNum = int(instanceNumF)
+		if instanceNum < 1 {
+			return MakeErrorResult("instance_num should be more than zero")
+		}
+		textCopy := text
+		countdown := instanceNum
+		pos := -1
+		l := len(oldText)
+		thrownTotal := 0
+		for {
+			countdown--
+			index := strings.Index(textCopy, oldText)
+			if index == -1 {
+				pos = -1
+				break
+			} else {
+				pos = index + thrownTotal
+				if countdown == 0 {
+					break
+				}
+				thrown := l + index
+				thrownTotal += thrown
+				textCopy = textCopy[thrown:]
+			}
+		}
+		if pos == -1 {
+			return MakeStringResult(text)
+		} else {
+			pre := text[:pos]
+			post := text[pos+l:]
+			return MakeStringResult(pre + newText + post)
+		}
+	}
+}
+
 // T is an implementation of the Excel T function that returns whether the
 // argument is text.
 func T(args []Result) Result {
@@ -752,3 +852,29 @@ func Text(args []Result) Result {
 		return MakeErrorResult("Incorrect argument for TEXT")
 	}
 }
+
+func getString(arg Result, funcName, argName string) (string, Result) {
+	switch arg.Type {
+	case ResultTypeString, ResultTypeNumber, ResultTypeEmpty:
+		return arg.Value(), empty
+	default:
+		return "", MakeErrorResult(funcName + " requires " + argName + " to be a number or string")
+	}
+}
+
+func getNumber(arg Result, funcName, argName string) (float64, Result) {
+	switch arg.Type {
+	case ResultTypeEmpty:
+		return 0, empty
+	case ResultTypeNumber:
+		return arg.ValueNumber, empty
+	case ResultTypeString:
+			f, err := strconv.ParseFloat(arg.ValueString, 64)
+			if err != nil {
+				return 0, MakeErrorResult(argName + " should be a number for " + funcName)
+			}
+			return f, empty
+	default:
+		return 0, MakeErrorResult(funcName + " requires " + argName + " to be a number or empty")
+	}
+}
diff --git a/spreadsheet/formula/functions_test.go b/spreadsheet/formula/functions_test.go
index 3429a581..020c659a 100644
--- a/spreadsheet/formula/functions_test.go
+++ b/spreadsheet/formula/functions_test.go
@@ -564,8 +564,29 @@ func TestMatch(t *testing.T) {
 	runTests(t, ctx, td)
 }
 
-func TestMaxA(t *testing.T) {
+func TestMax(t *testing.T) {
+	ss := spreadsheet.New()
+	sheet := ss.AddSheet()
 
+	sheet.Cell("A1").SetNumber(0.1)
+	sheet.Cell("B1").SetNumber(0.2)
+
+	sheet.Cell("A2").SetNumber(0.4)
+	sheet.Cell("B2").SetNumber(0.8)
+
+	sheet.Cell("A3").SetBool(true)
+	sheet.Cell("B3").SetBool(false)
+
+	ctx := sheet.FormulaContext()
+
+	td := []testStruct{
+		{`MAX(A1:B3)`, `0.8 ResultTypeNumber`},
+	}
+
+	runTests(t, ctx, td)
+}
+
+func TestMaxA(t *testing.T) {
 	ss := spreadsheet.New()
 	sheet := ss.AddSheet()
 
@@ -587,8 +608,29 @@ func TestMaxA(t *testing.T) {
 	runTests(t, ctx, td)
 }
 
-func TestMinA(t *testing.T) {
+func TestMin(t *testing.T) {
+	ss := spreadsheet.New()
+	sheet := ss.AddSheet()
 
+	sheet.Cell("A1").SetNumber(0.1)
+	sheet.Cell("B1").SetNumber(0.2)
+
+	sheet.Cell("A2").SetNumber(0.4)
+	sheet.Cell("B2").SetNumber(0.8)
+
+	sheet.Cell("A3").SetBool(true)
+	sheet.Cell("B3").SetBool(false)
+
+	ctx := sheet.FormulaContext()
+
+	td := []testStruct{
+		{`MIN(A1:B3)`, `0.1 ResultTypeNumber`},
+	}
+
+	runTests(t, ctx, td)
+}
+
+func TestMinA(t *testing.T) {
 	ss := spreadsheet.New()
 	sheet := ss.AddSheet()
 
@@ -611,7 +653,6 @@ func TestMinA(t *testing.T) {
 }
 
 func TestIfs(t *testing.T) {
-
 	ss := spreadsheet.New()
 	sheet := ss.AddSheet()
 
@@ -2742,3 +2783,111 @@ func TestYieldmat(t *testing.T) {
 
 	runTests(t, ctx, td)
 }
+
+func TestMid(t *testing.T) {
+	ss := spreadsheet.New()
+	sheet := ss.AddSheet()
+
+	ctx := sheet.FormulaContext()
+
+	td := []testStruct{
+		{`=MID("Fluid Flow",1,5)`, `Fluid ResultTypeString`},
+		{`=MID("Fluid Flow",7,20)`, `Flow ResultTypeString`},
+		{`=MID("Fluid Flow",20,5)`, ` ResultTypeString`},
+		{`=MID("Fluid Flow",1,0)`, ` ResultTypeString`},
+		{`=MID("Fluid Flow",0,5)`, `#VALUE! ResultTypeError`},
+		{`=MID("Fluid Flow",7,-20)`, `#VALUE! ResultTypeError`},
+	}
+
+	runTests(t, ctx, td)
+}
+
+func TestSubstitute(t *testing.T) {
+	ss := spreadsheet.New()
+	sheet := ss.AddSheet()
+
+	sheet.Cell("A1").SetString("Hello Earth Earth Earth")
+
+	ctx := sheet.FormulaContext()
+
+	td := []testStruct{
+		{`=SUBSTITUTE(A1,"Earth","Krypton",1)`, `Hello Krypton Earth Earth ResultTypeString`},
+		{`=SUBSTITUTE(A1,"Earth","Krypton",2)`, `Hello Earth Krypton Earth ResultTypeString`},
+		{`=SUBSTITUTE(A1,"Earth","Krypton",3)`, `Hello Earth Earth Krypton ResultTypeString`},
+		{`=SUBSTITUTE(A1,"Earth","Krypton",4)`, `Hello Earth Earth Earth ResultTypeString`},
+		{`=SUBSTITUTE(A1,"Earth","Krypton")`, `Hello Krypton Krypton Krypton ResultTypeString`},
+		{`=SUBSTITUTE(A1,"World","Krypton")`, `Hello Earth Earth Earth ResultTypeString`},
+		{`=SUBSTITUTE(A1,"Earth","Krypton",0)`, `#VALUE! ResultTypeError`},
+	}
+
+	runTests(t, ctx, td)
+}
+
+func TestAnd(t *testing.T) {
+	ss := spreadsheet.New()
+	sheet := ss.AddSheet()
+
+	ctx := sheet.FormulaContext()
+
+	td := []testStruct{
+		{`=AND(FALSE,FALSE)`, `0 ResultTypeNumber`},
+		{`=AND(TRUE,FALSE)`, `0 ResultTypeNumber`},
+		{`=AND(FALSE,TRUE)`, `0 ResultTypeNumber`},
+		{`=AND(TRUE,TRUE)`, `1 ResultTypeNumber`},
+	}
+
+	runTests(t, ctx, td)
+}
+
+func TestIferror(t *testing.T) {
+	ss := spreadsheet.New()
+	sheet := ss.AddSheet()
+
+	ctx := sheet.FormulaContext()
+
+	td := []testStruct{
+		{`=IFERROR("No error","ERROR")`, `No error ResultTypeString`},
+		{`=IFERROR(1/0,"ERROR")`, `ERROR ResultTypeString`},
+	}
+
+	runTests(t, ctx, td)
+}
+
+func TestChar(t *testing.T) {
+	ss := spreadsheet.New()
+	sheet := ss.AddSheet()
+
+	ctx := sheet.FormulaContext()
+
+	td := []testStruct{
+		{`=CHAR(65)`, `A ResultTypeString`},
+		{`=CHAR(255)`, `ÿ ResultTypeString`},
+		{`=CHAR(1000)`, `#VALUE! ResultTypeError`},
+		{`=CHAR("invalid")`, `#VALUE! ResultTypeError`},
+	}
+
+	runTests(t, ctx, td)
+}
+
+func TestRound(t *testing.T) {
+	ss := spreadsheet.New()
+	sheet := ss.AddSheet()
+
+	ctx := sheet.FormulaContext()
+
+	td := []testStruct{
+		{`=ROUND(2.14,1)`, `2.1 ResultTypeNumber`},
+		{`=ROUND(2.16,1)`, `2.2 ResultTypeNumber`},
+		{`=ROUND(-2.14,1)`, `-2.1 ResultTypeNumber`},
+		{`=ROUND(-2.16,1)`, `-2.2 ResultTypeNumber`},
+		{`=ROUND(21.5,-1)`, `20 ResultTypeNumber`},
+		{`=ROUND(21.5,-2)`, `0 ResultTypeNumber`},
+		{`=ROUND(-55.5,-1)`, `-60 ResultTypeNumber`},
+		{`=ROUND(-55.5,-2)`, `-100 ResultTypeNumber`},
+		{`=ROUND(-55.5,0)`, `-56 ResultTypeNumber`},
+		{`=ROUND(-55.4,)`, `-55 ResultTypeNumber`},
+		{`=ROUND(-55.4)`, `#VALUE! ResultTypeError`},
+	}
+
+	runTests(t, ctx, td)
+}