Pass format provider through to formatter

When using the default format string, callers still want to be able to
specify a formatter to make sure that dates are formatted in their
preferred way.

Ical.Net.Tests/CalDateTimeTests.cs

@@ -174,6 +174,14 @@ public static IEnumerable<ITestCaseData> ToStringTestCases()
             yield return new TestCaseData(new DateTime(2022, 8, 30), "o", null)
                 .Returns("2022-08-30T00:00:00.0000000+12:00 Pacific/Auckland")
                 .SetName("Date and time formatted using format string with no culture returns string using BCL formatter");
+
+            yield return new TestCaseData(new DateTime(2022, 8, 30, 10, 30, 0), null, CultureInfo.InvariantCulture)
+                .Returns("08/30/2022 10:30:00 Pacific/Auckland")
+                .SetName("Date and time with format provider");
+
+            yield return new TestCaseData(new DateTime(2022, 8, 30), null, CultureInfo.InvariantCulture)
+                .Returns("08/30/2022 Pacific/Auckland")
+                .SetName("Date only with current format provider");
         }
     }
 }

Ical.Net/DataTypes/CalDateTime.cs

@@ -538,13 +538,13 @@ public string ToString(string format, IFormatProvider formatProvider)
             }
             if (HasTime && HasDate)
             {
-                return Value + tz;
+                return Value.ToString(formatProvider) + tz;
             }
             if (HasTime)
             {
                 return Value.TimeOfDay + tz;
             }
-            return Value.ToString("d") + tz;
+            return Value.ToString("d", formatProvider) + tz;
         }
     }
 }