Add ToTimeInDefaultLocation/E

Go's time parsing uses UTC when the format doesn't have a tiemzone, and
has even weirder behavior when it has a zone name but no numeric offset.
A caller to `cast.ToTime` won't know if the returned time was explicitly
in UTC, or defaulted there, so the caller cannot fix it. These new
functions allow a user to supply a different timezone to default to,
with nil using the local zone.

cast.go

@@ -20,6 +20,11 @@ func ToTime(i interface{}) time.Time {
 	return v
 }
 
+func ToTimeInDefaultLocation(i interface{}, location *time.Location) time.Time {
+	v, _ := ToTimeInDefaultLocationE(i, location)
+	return v
+}
+
 // ToDuration casts an interface to a time.Duration type.
 func ToDuration(i interface{}) time.Duration {
 	v, _ := ToDurationE(i)

cast_test.go

@@ -1285,3 +1285,172 @@ func TestToDurationE(t *testing.T) {
 		assert.Equal(t, test.expect, v, errmsg)
 	}
 }
+
+func TestToTime(t *testing.T) {
+	est, err := time.LoadLocation("EST")
+	if !assert.NoError(t, err) {
+		return
+	}
+
+	irn, err := time.LoadLocation("Iran")
+	if !assert.NoError(t, err) {
+		return
+	}
+
+	swd, err := time.LoadLocation("Europe/Stockholm")
+	if !assert.NoError(t, err) {
+		return
+	}
+
+	// time.Parse*() fns handle the target & local timezones being the same
+	// differently, so make sure we use one of the timezones as local by
+	// temporarily change it.
+	if !locationEqual(time.Local, swd) {
+		var originalLocation *time.Location
+		originalLocation, time.Local = time.Local, swd
+		defer func() {
+			time.Local = originalLocation
+		}()
+	}
+
+	// Test same local time in different timezones
+	utc2016 := time.Date(2016, time.January, 1, 0, 0, 0, 0, time.UTC)
+	est2016 := time.Date(2016, time.January, 1, 0, 0, 0, 0, est)
+	irn2016 := time.Date(2016, time.January, 1, 0, 0, 0, 0, irn)
+	swd2016 := time.Date(2016, time.January, 1, 0, 0, 0, 0, swd)
+
+	for _, format := range timeFormats {
+		t.Logf("Checking time format '%s', has timezone: %v", format.format, format.hasTimezone)
+
+		est2016str := est2016.Format(format.format)
+		if !assert.NotEmpty(t, est2016str) {
+			continue
+		}
+
+		swd2016str := swd2016.Format(format.format)
+		if !assert.NotEmpty(t, swd2016str) {
+			continue
+		}
+
+		// Test conversion without a default location
+		converted, err := ToTimeE(est2016str)
+		if assert.NoError(t, err) {
+			if format.hasTimezone {
+				// Converting inputs with a timezone should preserve it
+				assertTimeEqual(t, est2016, converted)
+				assertLocationEqual(t, est, converted.Location())
+			} else {
+				// Converting inputs without a timezone should be interpreted
+				// as a local time in UTC.
+				assertTimeEqual(t, utc2016, converted)
+				assertLocationEqual(t, time.UTC, converted.Location())
+			}
+		}
+
+		// Test conversion of a time in the local timezone without a default
+		// location
+		converted, err = ToTimeE(swd2016str)
+		if assert.NoError(t, err) {
+			if format.hasTimezone {
+				// Converting inputs with a timezone should preserve it
+				assertTimeEqual(t, swd2016, converted)
+				assertLocationEqual(t, swd, converted.Location())
+			} else {
+				// Converting inputs without a timezone should be interpreted
+				// as a local time in UTC.
+				assertTimeEqual(t, utc2016, converted)
+				assertLocationEqual(t, time.UTC, converted.Location())
+			}
+		}
+
+		// Conversion with a nil default location sould have same behavior
+		converted, err = ToTimeInDefaultLocationE(est2016str, nil)
+		if assert.NoError(t, err) {
+			if format.hasTimezone {
+				// Converting inputs with a timezone should preserve it
+				assertTimeEqual(t, est2016, converted)
+				assertLocationEqual(t, est, converted.Location())
+			} else {
+				// Converting inputs without a timezone should be interpreted
+				// as a local time in the local timezone.
+				assertTimeEqual(t, swd2016, converted)
+				assertLocationEqual(t, swd, converted.Location())
+			}
+		}
+
+		// Test conversion with a default location that isn't UTC
+		converted, err = ToTimeInDefaultLocationE(est2016str, irn)
+		if assert.NoError(t, err) {
+			if format.hasTimezone {
+				// Converting inputs with a timezone should preserve it
+				assertTimeEqual(t, est2016, converted)
+				assertLocationEqual(t, est, converted.Location())
+			} else {
+				// Converting inputs without a timezone should be interpreted
+				// as a local time in the given location.
+				assertTimeEqual(t, irn2016, converted)
+				assertLocationEqual(t, irn, converted.Location())
+			}
+		}
+
+		// Test conversion of a time in the local timezone with a default
+		// location that isn't UTC
+		converted, err = ToTimeInDefaultLocationE(swd2016str, irn)
+		if assert.NoError(t, err) {
+			if format.hasTimezone {
+				// Converting inputs with a timezone should preserve it
+				assertTimeEqual(t, swd2016, converted)
+				assertLocationEqual(t, swd, converted.Location())
+			} else {
+				// Converting inputs without a timezone should be interpreted
+				// as a local time in the given location.
+				assertTimeEqual(t, irn2016, converted)
+				assertLocationEqual(t, irn, converted.Location())
+			}
+		}
+	}
+}
+
+func assertTimeEqual(t *testing.T, expected, actual time.Time, msgAndArgs ...interface{}) bool {
+	if !expected.Equal(actual) {
+		return assert.Fail(t, fmt.Sprintf("Expected time '%s', got '%s'", expected, actual), msgAndArgs...)
+	}
+	return true
+}
+
+func assertLocationEqual(t *testing.T, expected, actual *time.Location, msgAndArgs ...interface{}) bool {
+	if !locationEqual(expected, actual) {
+		return assert.Fail(t, fmt.Sprintf("Expected location '%s', got '%s'", expected, actual), msgAndArgs...)
+	}
+	return true
+}
+
+func locationEqual(a, b *time.Location) bool {
+	// A note about comparring time.Locations:
+	//   - can't only compare pointers
+	//   - can't compare loc.String() because locations with the same
+	//     name can have different offsets
+	//   - can't use reflect.DeepEqual because time.Location has internal
+	//     caches
+
+	if a == b {
+		return true
+	} else if a == nil || b == nil {
+		return false
+	}
+
+	// Check if they're equal by parsing times with a format that doesn't
+	// include a timezone, which will interpret it as being a local time in
+	// the given zone, and comparing the resulting local times.
+	tA, err := time.ParseInLocation("2006-01-02", "2016-01-01", a)
+	if err != nil {
+		return false
+	}
+
+	tB, err := time.ParseInLocation("2006-01-02", "2016-01-01", b)
+	if err != nil {
+		return false
+	}
+
+	return tA.Equal(tB)
+}

caste.go

@@ -20,13 +20,20 @@ var errNegativeNotAllowed = errors.New("unable to cast negative value")
 
 // ToTimeE casts an interface to a time.Time type.
 func ToTimeE(i interface{}) (tim time.Time, err error) {
+	return ToTimeInDefaultLocationE(i, time.UTC)
+}
+
+// ToTimeInDefaultLocationE casts an empty interface to time.Time,
+// interpreting inputs without a timezone to be in the given location,
+// or the local timezone if nil.
+func ToTimeInDefaultLocationE(i interface{}, location *time.Location) (tim time.Time, err error) {
 	i = indirect(i)
 
 	switch v := i.(type) {
 	case time.Time:
 		return v, nil
 	case string:
-		return StringToDate(v)
+		return StringToDateInDefaultLocation(v, location)
 	case int:
 		return time.Unix(int64(v), 0), nil
 	case int64:
@@ -1204,37 +1211,68 @@ func ToDurationSliceE(i interface{}) ([]time.Duration, error) {
 // predefined list of formats.  If no suitable format is found, an error is
 // returned.
 func StringToDate(s string) (time.Time, error) {
-	return parseDateWith(s, []string{
-		time.RFC3339,
-		"2006-01-02T15:04:05", // iso8601 without timezone
-		time.RFC1123Z,
-		time.RFC1123,
-		time.RFC822Z,
-		time.RFC822,
-		time.RFC850,
-		time.ANSIC,
-		time.UnixDate,
-		time.RubyDate,
-		"2006-01-02 15:04:05.999999999 -0700 MST", // Time.String()
-		"2006-01-02",
-		"02 Jan 2006",
-		"2006-01-02T15:04:05-0700", // RFC3339 without timezone hh:mm colon
-		"2006-01-02 15:04:05 -07:00",
-		"2006-01-02 15:04:05 -0700",
-		"2006-01-02 15:04:05Z07:00", // RFC3339 without T
-		"2006-01-02 15:04:05Z0700",  // RFC3339 without T or timezone hh:mm colon
-		"2006-01-02 15:04:05",
-		time.Kitchen,
-		time.Stamp,
-		time.StampMilli,
-		time.StampMicro,
-		time.StampNano,
-	})
+	return StringToDateInDefaultLocation(s, time.UTC)
+}
+
+// StringToDateInDefaultLocation casts an empty interface to a time.Time,
+// interpreting inputs without a timezone to be in the given location,
+// or the local timezone if nil.
+func StringToDateInDefaultLocation(s string, location *time.Location) (time.Time, error) {
+	if location == nil {
+		location = time.Local
+	}
+	return parseDateWith(s, location, timeFormats)
+}
+
+type timeFormat struct {
+	format      string
+	hasTimezone bool
 }
 
-func parseDateWith(s string, dates []string) (d time.Time, e error) {
-	for _, dateType := range dates {
-		if d, e = time.Parse(dateType, s); e == nil {
+var (
+	timeFormats = []timeFormat{
+		timeFormat{time.RFC3339, true},
+		timeFormat{"2006-01-02T15:04:05", false}, // iso8601 without timezone
+		timeFormat{time.RFC1123Z, true},
+		timeFormat{time.RFC1123, false},
+		timeFormat{time.RFC822Z, true},
+		timeFormat{time.RFC822, false},
+
+		timeFormat{time.RFC850, true},
+		timeFormat{"2006-01-02 15:04:05.999999999 -0700 MST", true}, // Time.String()
+		timeFormat{"2006-01-02T15:04:05-0700", true},                // RFC3339 without timezone hh:mm colon
+		timeFormat{"2006-01-02 15:04:05Z0700", true},                // RFC3339 without T or timezone hh:mm colon
+		timeFormat{"2006-01-02 15:04:05", false},
+
+		timeFormat{time.ANSIC, false},
+		timeFormat{time.UnixDate, false},
+		timeFormat{time.RubyDate, true},
+		timeFormat{"2006-01-02 15:04:05Z07:00", true},
+		timeFormat{"2006-01-02", false},
+		timeFormat{"02 Jan 2006", false},
+		timeFormat{"2006-01-02 15:04:05 -07:00", true},
+		timeFormat{"2006-01-02 15:04:05 -0700", true},
+		timeFormat{time.Kitchen, false},
+		timeFormat{time.Stamp, false},
+		timeFormat{time.StampMilli, false},
+		timeFormat{time.StampMicro, false},
+		timeFormat{time.StampNano, false},
+	}
+)
+
+func parseDateWith(s string, defaultLocation *time.Location, formats []timeFormat) (d time.Time, e error) {
+	for _, format := range formats {
+		if d, e = time.Parse(format.format, s); e == nil {
+
+			// Some time formats have a zone name, but no offset, so it gets
+			// put in that zone name (not the default one passed in to us), but
+			// without that zone's offset. So set the location manually.
+			if !format.hasTimezone && defaultLocation != nil {
+				year, month, day := d.Date()
+				hour, min, sec := d.Clock()
+				d = time.Date(year, month, day, hour, min, sec, d.Nanosecond(), defaultLocation)
+			}
+
 			return
 		}
 	}