fix: omit zone in "AllDay" event helpers

For a date-only event (i.e., an event that lasts for the full day) the
iCalendar specification indicates that the value for DTSTART / DTEND
should be a DATE

https://icalendar.org/iCalendar-RFC-5545/3-6-1-event-component.html

> The "VEVENT" is also the calendar component used to specify an
> anniversary or daily reminder within a calendar. These events have a
> DATE value type for the "DTSTART" property instead of the default value
> type of DATE-TIME. If such a "VEVENT" has a "DTEND" property, it MUST be
> specified as a DATE value also

The DATE format
(https://icalendar.org/iCalendar-RFC-5545/3-3-4-date.html) should omit
both time and zone/location elements and additionally notes that "The
"TZID" property parameter MUST NOT be applied to DATE properties"

As per the specification, this PR also adds an explicit "VALUE=DATE"
parameter when the AllDay helpers were called, to indicate that the
property's default value type has been overridden and the VEVENT is
intended to be an all-day event
https://icalendar.org/iCalendar-RFC-5545/3-2-20-value-data-types.html

Finally the SetDuration call has been updated to preserve the "AllDay"
characteristics if the existing start or end has been specified in DATE
format, which is also a requirement of the spec.

Contributes-to: #55

components.go

@@ -126,15 +126,23 @@ func (event *VEvent) SetStartAt(t time.Time, props ...PropertyParameter) {
 }
 
 func (event *VEvent) SetAllDayStartAt(t time.Time, props ...PropertyParameter) {
-	event.SetProperty(ComponentPropertyDtStart, t.UTC().Format(icalDateFormatUtc), props...)
+	event.SetProperty(
+		ComponentPropertyDtStart,
+		t.UTC().Format(icalDateFormatLocal),
+		append(props, WithValue(string(ValueDataTypeDate)))...,
+	)
 }
 
 func (event *VEvent) SetEndAt(t time.Time, props ...PropertyParameter) {
 	event.SetProperty(ComponentPropertyDtEnd, t.UTC().Format(icalTimestampFormatUtc), props...)
 }
 
 func (event *VEvent) SetAllDayEndAt(t time.Time, props ...PropertyParameter) {
-	event.SetProperty(ComponentPropertyDtEnd, t.UTC().Format(icalDateFormatUtc), props...)
+	event.SetProperty(
+		ComponentPropertyDtEnd,
+		t.UTC().Format(icalDateFormatLocal),
+		append(props, WithValue(string(ValueDataTypeDate)))...,
+	)
 }
 
 // SetDuration updates the duration of an event.
@@ -143,14 +151,29 @@ func (event *VEvent) SetAllDayEndAt(t time.Time, props ...PropertyParameter) {
 //
 // Notice: It will not set the DURATION key of the ics - only DTSTART and DTEND will be affected.
 func (event *VEvent) SetDuration(d time.Duration) error {
-	t, err := event.GetStartAt()
-	if err == nil {
-		event.SetEndAt(t.Add(d))
-		return nil
-	} else {
-		t, err = event.GetEndAt()
+	startProp := event.GetProperty(ComponentPropertyDtStart)
+	if startProp != nil {
+		t, err := event.GetStartAt()
 		if err == nil {
-			event.SetStartAt(t.Add(-d))
+			v, _ := startProp.parameterValue(ParameterValue)
+			if v == string(ValueDataTypeDate) {
+				event.SetAllDayEndAt(t.Add(d))
+			} else {
+				event.SetEndAt(t.Add(d))
+			}
+			return nil
+		}
+	}
+	endProp := event.GetProperty(ComponentPropertyDtEnd)
+	if endProp != nil {
+		t, err := event.GetEndAt()
+		if err == nil {
+			v, _ := endProp.parameterValue(ParameterValue)
+			if v == string(ValueDataTypeDate) {
+				event.SetAllDayStartAt(t.Add(-d))
+			} else {
+				event.SetStartAt(t.Add(-d))
+			}
 			return nil
 		}
 	}

components_test.go

@@ -8,6 +8,68 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+func TestSetAllDay(t *testing.T) {
+	date, _ := time.Parse(time.RFC822, time.RFC822)
+
+	testCases := []struct {
+		name     string
+		start    time.Time
+		end      time.Time
+		duration time.Duration
+		output   string
+	}{
+		{
+			name:  "test set all day - start",
+			start: date,
+			output: `BEGIN:VEVENT
+UID:test-allday
+DTSTART;VALUE=DATE:20060102
+END:VEVENT
+`,
+		},
+		{
+			name: "test set all day - end",
+			end:  date,
+			output: `BEGIN:VEVENT
+UID:test-allday
+DTEND;VALUE=DATE:20060102
+END:VEVENT
+`,
+		},
+		{
+			name:     "test set all day - duration",
+			start:    date,
+			duration: time.Hour * 24,
+			output: `BEGIN:VEVENT
+UID:test-allday
+DTSTART;VALUE=DATE:20060102
+DTEND;VALUE=DATE:20060103
+END:VEVENT
+`,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			e := NewEvent("test-allday")
+			if !tc.start.IsZero() {
+				e.SetAllDayStartAt(tc.start)
+			}
+			if !tc.end.IsZero() {
+				e.SetAllDayEndAt(tc.end)
+			}
+			if tc.duration != 0 {
+				err := e.SetDuration(tc.duration)
+				assert.NoError(t, err)
+			}
+
+			text := strings.ReplaceAll(e.Serialize(), "\r\n", "\n")
+
+			assert.Equal(t, tc.output, text)
+		})
+	}
+}
+
 func TestSetDuration(t *testing.T) {
 	date, _ := time.Parse(time.RFC822, time.RFC822)
 	duration := time.Duration(float64(time.Hour) * 2)

property.go

@@ -86,6 +86,17 @@ func trimUT8StringUpTo(maxLength int, s string) string {
 	return s[:length]
 }
 
+func (property *BaseProperty) parameterValue(param Parameter) (string, error) {
+	v, ok := property.ICalParameters[string(param)]
+	if !ok || len(v) == 0 {
+		return "", fmt.Errorf("parameter %q not found in property", param)
+	}
+	if len(v) != 1 {
+		return "", fmt.Errorf("expected only one value for parameter %q in property, found %d", param, len(v))
+	}
+	return v[0], nil
+}
+
 func (property *BaseProperty) serialize(w io.Writer) {
 	b := bytes.NewBufferString("")
 	fmt.Fprint(b, property.IANAToken)