深入理解GO时间处理(time.Time)

时间包括时间值和时区, 没有包含时区信息的时间是不完整的、有歧义的. 和外界传递或解析时间数据时, 应当像HTTP协议或unix-timestamp那样, 使用没有时区歧义的格式, 如果使用某些没有包含时区的非标准的时间表示格式(如yyyy-mm-dd HH:MM:SS), 是有隐患的, 因为解析时会使用场景的默认设置, 如系统时区, 数据库默认时区可能引发事故. 确保服务器系统、数据库、应用程序使用统一的时区, 如果因为一些历史原因, 应用程序各自保持着不同时区, 那么编程时要小心检查代码, 知道时间数据在使用不同时区的程序之间交换时的行为. 第三节会详细解释go程序在不同场景下time.Time的行为.

2. Time的数据结构

go1.9之前, time.Time的定义为

type Time struct {
	// sec gives the number of seconds elapsed since
	// January 1, year 1 00:00:00 UTC.
	sec int64

// nsec specifies a non-negative nanosecond
	// offset within the second named by Seconds.
	// It must be in the range [0, 999999999].
	nsec int32

// loc specifies the Location that should be used to
	// determine the minute, hour, month, day, and year
	// that correspond to this Time.
	// The nil location means UTC.
	// All UTC times are represented with loc==nil, never loc==&utcLoc.
	loc *Location
}

sec表示从公元1年1月1日00:00:00UTC到要表示的整数秒数, nsec表示余下的纳秒数, loc表示时区. sec和nsec处理没有歧义的时间值, loc处理偏移量.

因为2017年闰一秒, 国际时钟调整, Go程序两次取time.Now()相减的时间差得到了意料之外的负数, 导致cloudFlare的CDN服务中断, 详见https://blog.cloudflare.com/how-and-why-the-leap-second-affected-cloudflare-dns/, go1.9在不影响已有应用代码的情况下修改了time.Time的实现. go1.9的time.Time定义为

// A Time represents an instant in time with nanosecond precision.
//
// Programs using times should typically store and pass them as values,
// not pointers. That is, time variables and struct fields should be of
// type time.Time, not *time.Time.
//
// A Time value can be used by multiple goroutines simultaneously except
// that the methods GobDecode, UnmarshalBinary, UnmarshalJSON and
// UnmarshalText are not concurrency-safe.
//
// Time instants can be compared using the Before, After, and Equal methods.
// The Sub method subtracts two instants, producing a Duration.
// The Add method adds a Time and a Duration, producing a Time.
//
// The zero value of type Time is January 1, year 1, 00:00:00.000000000 UTC.
// As this time is unlikely to come up in practice, the IsZero method gives
// a simple way of detecting a time that has not been initialized explicitly.
//
// Each Time has associated with it a Location, consulted when computing the
// presentation form of the time, such as in the Format, Hour, and Year methods.
// The methods Local, UTC, and In return a Time with a specific location.
// Changing the location in this way changes only the presentation; it does not
// change the instant in time being denoted and therefore does not affect the
// computations described in earlier paragraphs.
//
// Note that the Go == operator compares not just the time instant but also the
// Location and the monotonic clock reading. Therefore, Time values should not
// be used as map or database keys without first guaranteeing that the
// identical Location has been set for all values, which can be achieved
// through use of the UTC or Local method, and that the monotonic clock reading
// has been stripped by setting t = t.Round(0). In general, prefer t.Equal(u)
// to t == u, since t.Equal uses the most accurate comparison available and
// correctly handles the case when only one of its arguments has a monotonic
// clock reading.
//
// In addition to the required “wall clock” reading, a Time may contain an optional
// reading of the current process's monotonic clock, to provide additional precision
// for comparison or subtraction.
// See the “Monotonic Clocks” section in the package documentation for details.
//
type Time struct {
	// wall and ext encode the wall time seconds, wall time nanoseconds,
	// and optional monotonic clock reading in nanoseconds.
	//
	// From high to low bit position, wall encodes a 1-bit flag (hasMonotonic),
	// a 33-bit seconds field, and a 30-bit wall time nanoseconds field.
	// The nanoseconds field is in the range [0, 999999999].
	// If the hasMonotonic bit is 0, then the 33-bit field must be zero
	// and the full signed 64-bit wall seconds since Jan 1 year 1 is stored in ext.
	// If the hasMonotonic bit is 1, then the 33-bit field holds a 33-bit
	// unsigned wall seconds since Jan 1 year 1885, and ext holds a
	// signed 64-bit monotonic clock reading, nanoseconds since process start.
	wall uint64
	ext  int64

// loc specifies the Location that should be used to
	// determine the minute, hour, month, day, and year
	// that correspond to this Time.
	// The nil location means UTC.
	// All UTC times are represented with loc==nil, never loc==&utcLoc.
	loc *Location
}

3. time的行为

1. 构造时间-获取现在时间-time.Now(), time.Now()使用本地时间, time.Local即本地时区, 取决于运行的系统环境设置, 优先取”TZ”这个环境变量, 然后取/etc/localtime, 都取不到就用UTC兜底.

   func Now() Time {
   	sec, nsec := now()
   	return Time{sec + unixToInternal, nsec, Local}
   }
   

1. 构造时间-获取某一时区的现在时间-time.Now().In(), Time结构体的In()方法仅设置loc, 不会改变时间值. 特别地, 如果是获取现在的UTC时间, 可以使用Time.Now().UTC().
   时区不能为nil. time包中只有两个时区变量time.Local和time.UTC. 其他时区变量有两种方法取得, 一个是通过time.LoadLocation函数根据时区名字加载, 时区名字见IANA Time Zone database, LoadLocation首先查找系统zoneinfo, 然后查找$GOROOT/lib/time/zoneinfo.zip.另一个是在知道时区名字和偏移量的情况下直接调用time.FixedZone("$zonename", $offsetSecond)构造一个Location对象.

   // In returns t with the location information set to loc.
   //
   // In panics if loc is nil.
   func (t Time) In(loc *Location) Time {
   	if loc == nil {
   		panic("time: missing Location in call to Time.In")
   	}
   	t.setLoc(loc)
   	return t
   }
   
   // LoadLocation returns the Location with the given name.
   //
   // If the name is "" or "UTC", LoadLocation returns UTC.
   // If the name is "Local", LoadLocation returns Local.
   //
   // Otherwise, the name is taken to be a location name corresponding to a file
   // in the IANA Time Zone database, such as "America/New_York".
   //
   // The time zone database needed by LoadLocation may not be
   // present on all systems, especially non-Unix systems.
   // LoadLocation looks in the directory or uncompressed zip file
   // named by the ZONEINFO environment variable, if any, then looks in
   // known installation locations on Unix systems,
   // and finally looks in $GOROOT/lib/time/zoneinfo.zip.
   func LoadLocation(name string) (*Location, error) {
   	if name == "" || name == "UTC" {
   		return UTC, nil
   	}
   	if name == "Local" {
   		return Local, nil
   	}
   	if zoneinfo != "" {
   		if z, err := loadZoneFile(zoneinfo, name); err == nil {
   			z.name = name
   			return z, nil
   		}
   	}
   	return loadLocation(name)
   }
   

1. 构造时间-手动构造时间-time.Date(), 传入年元日时分秒纳秒和时区变量Location构造一个时间. 得到的是指定location的时间.

   func Date(year int, month Month, day, hour, min, sec, nsec int, loc *Location) Time {
   	if loc == nil {
   		panic("time: missing Location in call to Date")
   	}
   .....
   }
   

1. 构造时间-从unix时间戳中构造时间, time.Unix(), 传入秒和纳秒构造.

2. 序列化反序列化时间-文本和JSON, fmt.Sprintf,fmt.SScanf, json.Marshal, json.Unmarshal时的, 使用的时间格式均包含时区信息, 序列化使用RFC3339Nano()”2006-01-02T15:04:05.999999999Z07:00”, 反序列化使用RFC3339()”2006-01-02T15:04:05Z07:00”, 反序列化没有纳秒值也可以正常序列化成功.

   // String returns the time formatted using the format string
   //	"2006-01-02 15:04:05.999999999 -0700 MST"
   func (t Time) String() string {
   	return t.Format("2006-01-02 15:04:05.999999999 -0700 MST")
   }
   // MarshalJSON implements the json.Marshaler interface.
   // The time is a quoted string in RFC 3339 format, with sub-second precision added if present.
   func (t Time) MarshalJSON() ([]byte, error) {
   	if y := t.Year(); y < 0 || y >= 10000 {
   		// RFC 3339 is clear that years are 4 digits exactly.
   		// See golang.org/issue/4556#c15 for more discussion.
   		return nil, errors.New("Time.MarshalJSON: year outside of range [0,9999]")
   	}
   
   b := make([]byte, 0, len(RFC3339Nano)+2)
   	b = append(b, '"')
   	b = t.AppendFormat(b, RFC3339Nano)
   	b = append(b, '"')
   	return b, nil
   }
   
   // UnmarshalJSON implements the json.Unmarshaler interface.
   // The time is expected to be a quoted string in RFC 3339 format.
   func (t *Time) UnmarshalJSON(data []byte) error {
   	// Ignore null, like in the main JSON package.
   	if string(data) == "null" {
   		return nil
   	}
   	// Fractional seconds are handled implicitly by Parse.
   	var err error
   	*t, err = Parse(`"`+RFC3339+`"`, string(data))
   	return err
   }
   

1. 序列化反序列化时间-HTTP协议中的date, 统一GMT, 代码位于net/http/server.go:878

   // TimeFormat is the time format to use when generating times in HTTP
   // headers. It is like time.RFC1123 but hard-codes GMT as the time
   // zone. The time being formatted must be in UTC for Format to
   // generate the correct format.
   //
   // For parsing this time format, see ParseTime.
   const TimeFormat = "Mon, 02 Jan 2006 15:04:05 GMT"
   

1. 序列化反序列化时间-time.Format("$layout"), time.Parse("$layout","$value"), time.ParseInLocation("$layout","$value","$Location")

   • time.Format("$layout")格式化时间时, 时区会参与计算. 调time.Time的Year()Month()Day()等获取年月日等时时区会参与计算, 得到一个使用偏移量修正过的正确的时间字符串, 若$layout有指定显示时区, 那么时区信息会体现在格式化后的时间字符串中. 如果$layout没有指定显示时区, 那么字符串只有时间没有时区, 时区是隐含的, time.Time对象中的时区.
   • time.Parse("$layout","$value"), 若$layout有指定显示时区, 那么时区信息会体现在格式化后的time.Time对象. 如果$layout没有指定显示时区, 那么使用会认为这是一个UTC时间, 时区是UTC.

   • time.ParseInLocation("$layout","$value","$Location") 使用传参的时区解析时间, 建议用这个, 没有歧义.

     // Parse parses a formatted string and returns the time value it represents.
     // The layout  defines the format by showing how the reference time,
     // defined to be
     //	Mon Jan 2 15:04:05 -0700 MST 2006
     // would be interpreted if it were the value; it serves as an example of
     // the input format. The same interpretation will then be made to the
     // input string.
     //
     // Predefined layouts ANSIC, UnixDate, RFC3339 and others describe standard
     // and convenient representations of the reference time. For more information
     // about the formats and the definition of the reference time, see the
     // documentation for ANSIC and the other constants defined by this package.
     // Also, the executable example for time.Format demonstrates the working
     // of the layout string in detail and is a good reference.
     //
     // Elements omitted from the value are assumed to be zero or, when
     // zero is impossible, one, so parsing "3:04pm" returns the time
     // corresponding to Jan 1, year 0, 15:04:00 UTC (note that because the year is
     // 0, this time is before the zero Time).
     // Years must be in the range 0000..9999. The day of the week is checked
     // for syntax but it is otherwise ignored.
     //
     // In the absence of a time zone indicator, Parse returns a time in UTC.
     //
     // When parsing a time with a zone offset like -0700, if the offset corresponds
     // to a time zone used by the current location (Local), then Parse uses that
     // location and zone in the returned time. Otherwise it records the time as
     // being in a fabricated location with time fixed at the given zone offset.
     //
     // No checking is done that the day of the month is within the month's
     // valid dates; any one- or two-digit value is accepted. For example
     // February 31 and even February 99 are valid dates, specifying dates
     // in March and May. This behavior is consistent with time.Date.
     //
     // When parsing a time with a zone abbreviation like MST, if the zone abbreviation
     // has a defined offset in the current location, then that offset is used.
     // The zone abbreviation "UTC" is recognized as UTC regardless of location.
     // If the zone abbreviation is unknown, Parse records the time as being
     // in a fabricated location with the given zone abbreviation and a zero offset.
     // This choice means that such a time can be parsed and reformatted with the
     // same layout losslessly, but the exact instant used in the representation will
     // differ by the actual zone offset. To avoid such problems, prefer time layouts
     // that use a numeric zone offset, or use ParseInLocation.
     func Parse(layout, value string) (Time, error) {
     	return parse(layout, value, UTC, Local)
     }
     
     // ParseInLocation is like Parse but differs in two important ways.
     // First, in the absence of time zone information, Parse interprets a time as UTC;
     // ParseInLocation interprets the time as in the given location.
     // Second, when given a zone offset or abbreviation, Parse tries to match it
     // against the Local location; ParseInLocation uses the given location.
     func ParseInLocation(layout, value string, loc *Location) (Time, error) {
     	return parse(layout, value, loc, loc)
     }
     
     func parse(layout, value string, defaultLocation, local *Location) (Time, error) {
     .....
     }
     

2. 序列化反序列化时间-go-sql-driver/mysql中的时间处理.
   MySQL驱动解析时间的前提是连接字符串加了parseTime和loc, 如果parseTime为false, 会把mysql的date类型变成[]byte/string自行处理, parseTime为true才处理时间, loc指定MySQL中存储时间数据的时区, 如果没有指定loc, 用UTC. 序列化和反序列化均使用连接字符串中的设定的loc, SQL语句中的time.Time类型的参数的时区信息如果和loc不同, 则会调用t.In(loc)方法转时区.

   • 解析连接字符串的代码位于parseDSNParams函数https://github.com/go-sql-driver/mysql/blob/master/dsn.go#L467-L490

     	// Time Location
     case "loc":
     	if value, err = url.QueryUnescape(value); err != nil {
     		return
     	}
     	cfg.Loc, err = time.LoadLocation(value)
     	if err != nil {
     		return
     	}
     // time.Time parsing
     case "parseTime":
     	var isBool bool
     	cfg.ParseTime, isBool = readBool(value)
     	if !isBool {
     		return errors.New("invalid bool value: " + value)
     	}
     

   • 解析SQL语句中time.Time类型的参数的代码位于mysqlConn.interpolateParams方法https://github.com/go-sql-driver/mysql/blob/master/connection.go#L230-L273

     case time.Time:
     	if v.IsZero() {
     		buf = append(buf, "'0000-00-00'"...)
     	} else {
     		v := v.In(mc.cfg.Loc)
     		v = v.Add(time.Nanosecond * 500) // To round under microsecond
     		year := v.Year()
     		year100 := year / 100
     		year1 := year % 100
     		month := v.Month()
     		day := v.Day()
     		hour := v.Hour()
     		minute := v.Minute()
     		second := v.Second()
     		micro := v.Nanosecond() / 1000
     
     buf = append(buf, []byte{
     			'\'',
     			digits10[year100], digits01[year100],
     			digits10[year1], digits01[year1],
     			'-',
     			digits10[month], digits01[month],
     			'-',
     			digits10[day], digits01[day],
     			' ',
     			digits10[hour], digits01[hour],
     			':',
     			digits10[minute], digits01[minute],
     			':',
     			digits10[second], digits01[second],
     		}...)
     
     if micro != 0 {
     			micro10000 := micro / 10000
     			micro100 := micro / 100 % 100
     			micro1 := micro % 100
     			buf = append(buf, []byte{
     				'.',
     				digits10[micro10000], digits01[micro10000],
     				digits10[micro100], digits01[micro100],
     				digits10[micro1], digits01[micro1],
     			}...)
     		}
     		buf = append(buf, '\'')
     	}
     

   • 从MySQL数据流中解析时间的代码位于textRows.readRow方法https://github.com/go-sql-driver/mysql/blob/master/packets.go#L772-L777, 注意只要MySQL连接字符串设置了parseTime=true, 就会解析时间, 不管你是用string还是time.Time接收的.

     if !isNull {
     				if !mc.parseTime {
     					continue
     				} else {
     					switch rows.rs.columns[i].fieldType {
     					case fieldTypeTimestamp, fieldTypeDateTime,
     						fieldTypeDate, fieldTypeNewDate:
     						dest[i], err = parseDateTime(
     							string(dest[i].([]byte)),
     							mc.cfg.Loc,
     						)
     						if err == nil {
     							continue
     						}
     					default:
     						continue
     					}
     				}
     
     }
     

4. time时区处理不当案例

1. 有个服务频繁使用最新汇率, 所以缓存了最新汇率对象, 汇率对象的过期时间设为第二天北京时间零点, 汇率过期则从数据库中去最新汇率, 设置过期时间的代码如下:

   var startTime string = time.Now().UTC().Add(8 * 	time.Hour).Format("2006-01-02")
   tm2, _ := time.Parse("2006-01-02", startTime)
   lastTime = tm2.Unix() + 24*60*60
   

   这段代码使用了time.Parse, 如果时间格式中没有指定时区, 那么会得到使用本地时区下的第二天零点, 服务器时区设置为UTC0, 于是汇率缓存在UTC零点即北京时间八点才更新.

2. 公共库中有一个GetBjTime()方法, 注释写着将服务器UTC转成北京时间, 代码如下

   // 原版
   func GetBjTime() time.Time {
   // 将服务器UTC转成北京时间
   uTime := time.Now().UTC()
   dur, _ := time.ParseDuration("+8h")
   return uTime.Add(dur)
   }
   // 改
   func GetBjTime() time.Time {
   // 将服务器UTC转成北京时间
   uTime := time.Now()
   return uTime.In(time.FixedZone("CST", 8*60*60))
   }
   

   同事用这个方法将得到的time.Time参与计算, 发现多了8个小时. 觉得有问题, 同事和我讨论了之后, 我们得出结论后就大意地直接把原有函数改了, 我们都没有意识到这是个非常危险操作, 只所以危险是因为这个函数已经在很多服务的代码里用着(要稳!不能乱动公共库!!!). 之前用这个函数是因为老Java项目运行在时区为东八区的系统上, 大量代码使用东八区时间, 但数据库MySQL时区设置为UTC, go项目也运行在UTC时区. 也就是说, Java项目在把时区为UTC数据库当做是东八区来用, Java程序往MySQL写东八区的时间字符串, 在sequel软件中看表内容时虽然字符串是一样的, 但其实内部是UTC的时间, go代码的mysql连接字符串中loc选项为空, 就会使用UTC时区去解析数据, 拿到的数据会多八个小时. 例如Java代码往mysql插入一条”2017-10-29 22:00:00”数据本意是东八区2017年10月29日22点, 但在MySQL内部看来, 这是UTC的2017年10月29日22点, 换算成东八区时间为2017年10月30日6点, 如果其它程序解析时认为时间数据是MySQL的UTC时区, 那么会得到一个错误的时间. 所以才会在GO中要往Java代码创建的表写入数据时用time.Now().UTC().Add(time.Hour*8)直接相加八小时使得Java项目行为一致, 拿UTC的数据库存东八区时间.

   后面想想, 面对这种数据库中有时区不一致数据的情况, 在没有办法统一UTC时区的情况下, 应当使用MySQL时间字符串而不是time.Time来传递以避免时区隐含转换问题, 写入时参数传string类型的时间字符串, 解析时先拿到时间字符串, 然后自行判断建表时这个字段用的是东八区的时间字符串还是UTC时间字符串进行time.ParseInLocation得到时间对象, MySQL连接字符串的parseTime选项要设置为false. 比如我想在MySQL中存东八区的当前时间, SQL参数用Format后的字符串而不是传time.Time, 原版的time.Now().UTC().Add(time.Hour*8).Format("2006-01-02 15:04:05")和修改的time.Now().In(time.FixedZone("CST", 8*60*60))的输出将是一样, 但后者是正确的东八区现在时间. 原版的GetBjTime()返回time.Time可能用GetBeijingNowTimeString返回string更能体现本意吧.

5. 时间有关的标准

• UTC

  协调世界时（英语：Coordinated Universal Time，法语：Temps Universel Coordonné，简称UTC）是最主要的世界时间标准，其以原子时秒长为基础，在时刻上尽量接近于格林尼治标准时间。中华民国采用CNS 7648的《资料元及交换格式–资讯交换–日期及时间的表示法》（与ISO 8601类似）称之为世界协调时间。中华人民共和国采用ISO 8601:2000的国家标准GB/T 7408-2005《数据元和交换格式 信息交换 日期和时间表示法》中亦称之为协调世界时。
  协调世界时是世界上调节时钟和时间的主要时间标准，它与0度经线的平太阳时相差不超过1秒[4]，并不遵守夏令时。协调世界时是最接近格林威治标准时间(GMT)的几个替代时间系统之一。对于大多数用途来说，UTC时间被认为能与GMT时间互换，但GMT时间已不再被科学界所确定。

• ISO 8601 计算某一天在一年的第几周/循环时间RRlue/会用到此标准

  国际标准ISO 8601，是国际标准化组织的日期和时间的表示方法，全称为《数据存储和交换形式·信息交换·日期和时间的表示方法》。目前是第三版“ISO8601:2004”以替代第一版“ISO8601:1988”与第二版“ISO8601:2000”。

• UNIX时间

  UNIX时间，或称POSIX时间是UNIX或类UNIX系统使用的时间表示方式：从协调世界时1970年1月1日0时0分0秒起至现在的总秒数，不考虑闰秒[1]。 在多数Unix系统上Unix时间可以通过date +%s指令来检查。

• 时区