time: add DateTime, DateOnly, and TimeOnly

Add named constants for the 3rd, 4th, and 13th most popular formats.

Fixes #52746

Change-Id: I7ce92e44dcae18c089124f1d6f5bc2d6359d436c
Reviewed-on: https://go-review.googlesource.com/c/go/+/412495
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Joseph Tsai <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Joseph Tsai <[email protected]>

Author: dsnet

api/next/52746.txt

@@ -0,0 +1,6 @@
+pkg time, const DateOnly = "2006-01-02" #52746
+pkg time, const DateOnly ideal-string #52746
+pkg time, const DateTime = "2006-01-02 15:04:05" #52746
+pkg time, const DateTime ideal-string #52746
+pkg time, const TimeOnly = "15:04:05" #52746
+pkg time, const TimeOnly ideal-string #52746

src/time/format.go

@@ -116,6 +116,9 @@ const (
 	StampMilli = "Jan _2 15:04:05.000"
 	StampMicro = "Jan _2 15:04:05.000000"
 	StampNano  = "Jan _2 15:04:05.000000000"
+	DateTime   = "2006-01-02 15:04:05"
+	DateOnly   = "2006-01-02"
+	TimeOnly   = "15:04:05"
 )
 
 const (

src/time/format_test.go

@@ -115,6 +115,9 @@ var formatTests = []FormatTest{
 	{"StampMilli", StampMilli, "Feb  4 21:00:57.012"},
 	{"StampMicro", StampMicro, "Feb  4 21:00:57.012345"},
 	{"StampNano", StampNano, "Feb  4 21:00:57.012345600"},
+	{"DateTime", DateTime, "2009-02-04 21:00:57"},
+	{"DateOnly", DateOnly, "2009-02-04"},
+	{"TimeOnly", TimeOnly, "21:00:57"},
 	{"YearDay", "Jan  2 002 __2 2", "Feb  4 035  35 4"},
 	{"Year", "2006 6 06 _6 __6 ___6", "2009 6 09 _6 __6 ___6"},
 	{"Month", "Jan January 1 01 _1", "Feb February 2 02 _2"},