fix: normalize datetime format for Graph API event creation

Graph API requires dateTime without timezone offset when timeZone field
is set. Added parseFlexibleDateTime helper that accepts multiple input
formats (RFC3339, ISO, date+space+time) and strips the offset.

Fixes HTTP 400 'unable to deserialize' on cal create.

Author: lcorneliussen

internal/cal/cal.go

@@ -135,6 +135,40 @@ func List(cfg *config.Config, fromDate, toDate time.Time, search, account string
 	return nil
 }
 
+// parseFlexibleDateTime parses various datetime formats and converts to the configured timezone
+func parseFlexibleDateTime(input, timezoneName string) (string, error) {
+	loc, err := time.LoadLocation(timezoneName)
+	if err != nil {
+		return "", fmt.Errorf("failed to load timezone %s: %w", timezoneName, err)
+	}
+
+	// Try parsing various formats
+	formats := []string{
+		time.RFC3339,                // "2026-03-04T10:00:00+01:00"
+		"2006-01-02T15:04:05",       // "2026-03-04T10:00:00"
+		"2006-01-02 15:04",          // "2026-03-04 10:00"
+	}
+
+	var parsed time.Time
+	for _, format := range formats {
+		t, err := time.Parse(format, input)
+		if err == nil {
+			parsed = t
+			break
+		}
+	}
+
+	if parsed.IsZero() {
+		return "", fmt.Errorf("unable to parse datetime: %s", input)
+	}
+
+	// Convert to configured timezone
+	inZone := parsed.In(loc)
+
+	// Format without offset for Graph API
+	return inZone.Format("2006-01-02T15:04:05.0000000"), nil
+}
+
 // Create creates a new calendar event
 func Create(cfg *config.Config, account, subject, start, end, location, body string, attendees []string, force bool) error {
 	// Check cross-tenant unless force is enabled
@@ -150,17 +184,28 @@ func Create(cfg *config.Config, account, subject, start, end, location, body str
 		return err
 	}
 
+	// Parse and convert datetimes to configured timezone
+	startDateTime, err := parseFlexibleDateTime(start, cfg.Timezone)
+	if err != nil {
+		return fmt.Errorf("invalid start datetime: %w", err)
+	}
+
+	endDateTime, err := parseFlexibleDateTime(end, cfg.Timezone)
+	if err != nil {
+		return fmt.Errorf("invalid end datetime: %w", err)
+	}
+
 	// Create event
 	client := graph.NewClient(token)
 
 	event := &graph.Event{
 		Subject: subject,
 		Start: graph.DateTime{
-			DateTime: start,
+			DateTime: startDateTime,
 			TimeZone: cfg.Timezone,
 		},
 		End: graph.DateTime{
-			DateTime: end,
+			DateTime: endDateTime,
 			TimeZone: cfg.Timezone,
 		},
 	}