你真的了解 time.Now() 吗？

// 为了减少文章长度突出重点，注释部分有所删改

// A Time represents an instant in time with nanosecond precision.
// 
// The zero value of type Time is January 1, year 1, 00:00:00.000000000 UTC.
//
// 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.
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

	// 本文重点不在时区，或另写一篇文章讨论相关话题
	loc *Location
}

// Now returns the current local time.
func Now() Time {
	sec, nsec, mono := now()
	mono -= startNano
	sec += unixToInternal - minWall
	if uint64(sec)>>33 != 0 {
		// Seconds field overflowed the 33 bits available when
		// storing a monotonic time. This will be true after
		// March 16, 2157.
		return Time{uint64(nsec), sec + minWall, Local}
	}
	return Time{hasMonotonic | uint64(sec)<<nsecShift | uint64(nsec), mono, Local}
}

// Provided by package runtime.
func now() (sec int64, nsec int32, mono int64)

// Unix returns the local Time corresponding to the given Unix time,
// sec seconds and nsec nanoseconds since January 1, 1970 UTC.
// It is valid to pass nsec outside the range [0, 999999999].
// Not all sec values have a corresponding time value. One such
// value is 1<<63-1 (the largest int64 value).
func Unix(sec int64, nsec int64) Time {
	if nsec < 0 || nsec >= 1e9 {
		n := nsec / 1e9
		sec += n
		nsec -= n * 1e9
		if nsec < 0 {
			nsec += 1e9
			sec--
		}
	}
	return unixTime(sec, int32(nsec))
}

func unixTime(sec int64, nsec int32) Time {
	return Time{uint64(nsec), sec + unixToInternal, Local}
}

const (
	unixToInternal int64 = (1969*365 + 1969/4 - 1969/100 + 1969/400) * secondsPerDay
)

// UnixMilli returns the local Time corresponding to the given Unix time,
// msec milliseconds since January 1, 1970 UTC.
func UnixMilli(msec int64) Time {
	return Unix(msec/1e3, (msec%1e3)*1e6)
}

// UnixMicro returns the local Time corresponding to the given Unix time,
// usec microseconds since January 1, 1970 UTC.
func UnixMicro(usec int64) Time {
	return Unix(usec/1e6, (usec%1e6)*1e3)
}

// 为了减少文章长度突出重点，代码和注释部分有所删改

// Date returns the Time corresponding to
//
//	yyyy-mm-dd hh:mm:ss + nsec nanoseconds
//
// The month, day, hour, min, sec, and nsec values may be outside
// their usual ranges and will be normalized during the conversion.
// For example, October 32 converts to November 1.
func Date(year int, month Month, day, hour, min, sec, nsec int, loc *Location) Time {
	...

	// Normalize overflow
	...

	// Compute days since the absolute epoch.
	d := daysSinceEpoch(year)

	// Add in days before this month.
	d += uint64(daysBefore[month-1])
	if isLeap(year) && month >= March {
		d++ // February 29
	}

	// Add in days before today.
	d += uint64(day - 1)

	// Add in time elapsed today.
	abs := d * secondsPerDay
	abs += uint64(hour*secondsPerHour + min*secondsPerMinute + sec)

	unix := int64(abs) + (absoluteToInternal + internalToUnix)

	// Look for zone offset for expected time, so we can adjust to UTC.
	...

	t := unixTime(unix, int32(nsec))
	t.setLoc(loc)
	return t
}

// 为了减少文章长度突出重点，代码和注释部分有所删改

func Parse(layout, value string) (Time, error) {
	return parse(layout, value, UTC, Local)
}

func parse(layout, value string, defaultLocation, local *Location) (Time, error) {
	...
	return Date(year, Month(month), day, hour, min, sec, nsec, defaultLocation), nil
}

// Sub returns the duration t-u. If the result exceeds the maximum (or minimum)
// value that can be stored in a Duration, the maximum (or minimum) duration
// will be returned.
// To compute t-d for a duration d, use t.Add(-d).
func (t Time) Sub(u Time) Duration {
	if t.wall&u.wall&hasMonotonic != 0 {
		te := t.ext
		ue := u.ext
		d := Duration(te - ue)
		if d < 0 && te > ue {
			return maxDuration // t - u is positive out of range
		}
		if d > 0 && te < ue {
			return minDuration // t - u is negative out of range
		}
		return d
	}
	d := Duration(t.sec()-u.sec())*Second + Duration(t.nsec()-u.nsec())
	// Check for overflow or underflow.
	switch {
	case u.Add(d).Equal(t):
		return d // d is correct
	case t.Before(u):
		return minDuration // t - u is negative out of range
	default:
		return maxDuration // t - u is positive out of range
	}
}

var t time.Time

// 为了减少文章长度突出重点，增加了部分注释

// IsZero reports whether t represents the zero time instant,
// January 1, year 1, 00:00:00 UTC.
func (t Time) IsZero() bool {
	return t.sec() == 0 && t.nsec() == 0
}

// sec returns the time's seconds since Jan 1 year 1.
func (t *Time) sec() int64 {
	if t.wall&hasMonotonic != 0 { // 0 值时不会走到这个分支
		return wallToInternal + int64(t.wall<<1>>(nsecShift+1))
	}
	return t.ext
}

// nsec returns the time's nanoseconds.
// 就是简单的返回 wall 的第三部分（30 位纳秒时间偏移）
func (t *Time) nsec() int32 {
	return int32(t.wall & nsecMask)
}

const nsecMask = 1<<30 - 1

// Equal reports whether t and u represent the same time instant.
// Two times can be equal even if they are in different locations.
// For example, 6:00 +0200 and 4:00 UTC are Equal.
// See the documentation on the Time type for the pitfalls of using == with
// Time values; most code should use Equal instead.
func (t Time) Equal(u Time) bool {
	if t.wall&u.wall&hasMonotonic != 0 {
		return t.ext == u.ext
	}
	return t.sec() == u.sec() && t.nsec() == u.nsec()
}

// Provided by package runtime.
func now() (sec int64, nsec int32, mono int64)

//go:build !faketime && (windows || (linux && amd64))

//go:linkname time_now time.now
func time_now() (sec int64, nsec int32, mono int64)

//go:build !faketime && !windows && !(linux && amd64)

//go:linkname time_now time.now
func time_now() (sec int64, nsec int32, mono int64) {
	sec, nsec = walltime()
	return sec, nsec, nanotime()
}

// 为了减少文章长度突出重点，代码和注释部分有所删改

// func time.now() (sec int64, nsec int32, mono int64)
// $16-24 表示函数需要 16 字节的栈空间和 24 字节的返回值空间
TEXT time·now<ABIInternal>(SB),NOSPLIT,$16-24
	// 准备 vDSO
    ...

	// （如果不是 g0）切换到 g0
	...

noswitch: // 获取时间的逻辑（优先利用 vDSO）
	SUBQ	$32, SP		// Space for two time results
	ANDQ	$~15, SP	// Align for C code

	MOVL	$0, DI // CLOCK_REALTIME（获取日历时钟）
	LEAQ	16(SP), SI
	MOVQ	runtime·vdsoClockgettimeSym(SB), AX
	CMPQ	AX, $0
	JEQ	fallback // 失败跳转
	CALL	AX

	MOVL	$1, DI // CLOCK_MONOTONIC（获取单调钟）
	LEAQ	0(SP), SI
	MOVQ	runtime·vdsoClockgettimeSym(SB), AX
	CALL	AX

ret: // 获取成功，结果存在了栈中
	MOVQ	16(SP), AX	// realtime sec
	MOVQ	24(SP), DI	// realtime nsec (moved to BX below)
	MOVQ	0(SP), CX	// monotonic sec
	IMULQ	$1000000000, CX
	MOVQ	8(SP), DX	// monotonic nsec

	// 恢复现场
    ...

	// 返回结果
	// set result registers; AX is already correct
	MOVQ	DI, BX
	ADDQ	DX, CX // 计算出 monotonic nanoseconds
	RET

fallback: // CLOCK_REALTIME 获取失败时，证明 vdso 失败，利用系统调用获取
	MOVQ	$SYS_clock_gettime, AX
	SYSCALL

	MOVL	$1, DI // CLOCK_MONOTONIC
	LEAQ	0(SP), SI
	MOVQ	$SYS_clock_gettime, AX
	SYSCALL

	JMP	ret

func walltime() (sec int64, nsec int32)

//go:nosplit
func nanotime() int64 {
	return nanotime1()
}

func nanotime1() int64

import (
	_ "unsafe"
)

//go:noescape
//go:linkname Monotonic runtime.nanotime
func Monotonic() int64

// Round returns the result of rounding t to the nearest multiple of d (since the zero time).
// The rounding behavior for halfway values is to round up.
// If d <= 0, Round returns t stripped of any monotonic clock reading but otherwise unchanged.
//
// Round operates on the time as an absolute duration since the
// zero time; it does not operate on the presentation form of the
// time. Thus, Round(Hour) may return a time with a non-zero
// minute, depending on the time's Location.
func (t Time) Round(d Duration) Time {
	t.stripMono()
	if d <= 0 {
		return t
	}
	_, r := div(t, d)
	if lessThanHalf(r, d) {
		return t.Add(-r)
	}
	return t.Add(d - r)
}

// stripMono strips the monotonic clock reading in t.
func (t *Time) stripMono() {
	if t.wall&hasMonotonic != 0 {
		t.ext = t.sec()
		t.wall &= nsecMask
	}
}