datetime: always return time with fixed zone

Before the patch there was a double behavior:

1. If a user create Datetime from time.Time then Datetime.ToTime()
return an original time value with an original timezone.
2. If an Tarantool returns Datetime then we use a timezone with a
fixed offset. We need it to avoid changes of the timezone if a user
create a new Datetime from a Datetime.ToTime() value.

As a result, it was was possible to get different behaviors when
a user use time.Time.AddDate() or time.Time.Add() for DST timezones.

One possible solution is to always return a time with a fixed offset
from Datetime.ToTime() and recommend to use Datetime.Add() or
Datetime.Sub() for change time values.

Author: oleg-jukovec

CHANGELOG.md

@@ -16,6 +16,8 @@ Versioning](http://semver.org/spec/v2.0.0.html) except to the first release.
 
 ### Changed
 
+- Datetime.ToTime() always returns a time with a fixed timezone (#217)
+
 ### Fixed
 
 - Mode type description in the connection_pool subpackage (#208)

datetime/datetime.go

@@ -109,6 +109,7 @@ func NewDatetime(t time.Time) (*Datetime, error) {
 			return nil, fmt.Errorf("unknown timezone %s with offset %d",
 				zone, offset)
 		}
+		t = t.In(time.FixedZone(zone, offset))
 	}
 	if offset < offsetMin || offset > offsetMax {
 		return nil, fmt.Errorf("offset must be between %d and %d hours",
@@ -192,11 +193,32 @@ func (dtime *Datetime) add(ival Interval, positive bool) (*Datetime, error) {
 	newVal.Sec += direction * ival.Sec
 	newVal.Nsec += direction * ival.Nsec
 
+	newLoc := dtime.time.Location()
+	zone, _ := dtime.time.Zone()
+	locChanged := false
+
+	if loc, err := time.LoadLocation(zone); err == nil {
+		newLoc = loc
+		locChanged = true
+	}
+
 	tm := time.Date(int(newVal.Year), time.Month(newVal.Month),
 		int(newVal.Day), int(newVal.Hour), int(newVal.Min),
-		int(newVal.Sec), int(newVal.Nsec), dtime.time.Location())
+		int(newVal.Sec), int(newVal.Nsec), newLoc)
+
+	seconds := tm.Unix()
+	if seconds < minSeconds || seconds > maxSeconds {
+		return nil, fmt.Errorf("time %s is out of supported range", tm)
+	}
+
+	if locChanged {
+		_, offset := tm.Zone()
+		tm = tm.In(time.FixedZone(zone, offset))
+	}
 
-	return NewDatetime(tm)
+	dt := new(Datetime)
+	dt.time = tm
+	return dt, nil
 }
 
 // Add creates a new Datetime as addition of the Datetime and Interval. It may

datetime/example_test.go

@@ -165,6 +165,35 @@ func ExampleDatetime_Add() {
 	// New time: 2014-02-28 17:57:29.000000009 +0000 UTC
 }
 
+// ExampleDatetime_Add_dst demonstrates how to add an Interval to a Datetime value.
+func ExampleDatetime_Add_dst() {
+	loc := time.FixedZone("Europe/Moscow", 3*60*60)
+	tm := time.Date(2008, 1, 1, 1, 1, 1, 1, loc)
+	dt, err := NewDatetime(tm)
+	if err != nil {
+		fmt.Printf("Unable to create Datetime: %s", err)
+	}
+
+	fmt.Printf("Datime time:\n")
+	fmt.Printf("%s\n", dt.ToTime())
+	fmt.Printf("Datime time + 6 month:\n")
+	fmt.Printf("%s\n", dt.ToTime().AddDate(0, 6, 0))
+	dt, err = dt.Add(Interval{Month: 6})
+	if err != nil {
+		fmt.Printf("Unable to add 6 month: %s", err)
+	}
+	fmt.Printf("Datetime + 6 month time:\n")
+	fmt.Printf("%s\n", dt.ToTime())
+
+	// Output:
+	// Datime time:
+	// 2008-01-01 01:01:01.000000001 +0300 Europe/Moscow
+	// Datime time + 6 month:
+	// 2008-07-01 01:01:01.000000001 +0300 Europe/Moscow
+	// Datetime + 6 month time:
+	// 2008-07-01 01:01:01.000000001 +0400 Europe/Moscow
+}
+
 // ExampleDatetime_Sub demonstrates how to subtract an Interval from a
 // Datetime value.
 func ExampleDatetime_Sub() {
@@ -235,3 +264,42 @@ func ExampleInterval_Sub() {
 	// Output:
 	// {-9 2 3 0 0 -30 10 0 1}
 }
+
+// Example_ToTime_fixedZone demonstrates that produced time by Datetime
+// has fixed timezone. The zone is selected from the original value.
+func ExampleDatetime_ToTime_fixedZone() {
+	loc, err := time.LoadLocation("Europe/Moscow")
+	if err != nil {
+		fmt.Printf("Unable to load Europe/Moscow: %s", err)
+		return
+	}
+
+	tm := time.Date(2008, 1, 1, 1, 1, 1, 1, loc)
+	dt, err := NewDatetime(tm)
+	if err != nil {
+		fmt.Printf("Unable to create datetime with custom location: %s", err)
+	}
+
+	fmt.Printf("Original time:\n")
+	fmt.Printf("%s\n", tm)
+	fmt.Printf("Produced time:\n")
+	fmt.Printf("%s\n", dt.ToTime())
+	fmt.Printf("Original time + 6 month:\n")
+	fmt.Printf("%s\n", tm.AddDate(0, 6, 0))
+	fmt.Printf("Produced time + 6 month:\n")
+	fmt.Printf("%s\n", dt.ToTime().AddDate(0, 6, 0))
+	fmt.Printf("Datetime + 6 month:\n")
+	dt.Add(Interval{Month: 6})
+	fmt.Printf("%s\n", dt.ToTime())
+	// Output:
+	// Original time:
+	// 2008-01-01 01:01:01.000000001 +0300 MSK
+	// Produced time:
+	// 2008-01-01 01:01:01.000000001 +0300 MSK
+	// Original time + 6 month:
+	// 2008-07-01 01:01:01.000000001 +0400 MSD
+	// Produced time + 6 month:
+	// 2008-07-01 01:01:01.000000001 +0300 MSK
+	// Datetime + 6 month:
+	// 2008-01-01 01:01:01.000000001 +0300 MSK
+}