Support decimal precision timestamps (#3376)

Adds support for decimal precision UNIX timestamps up to thousandths of a second

CHANGELOG_PENDING.md

@@ -1,5 +1,6 @@
 ### SDK Features
 
 ### SDK Enhancements
+* `private/protocol`: Adds support for decimal precision UNIX timestamps up to thousandths of a second ([#3376](https://github.com/aws/aws-sdk-go/pull/3376))
 
 ### SDK Bugs

private/protocol/timestamp.go

@@ -56,7 +56,8 @@ func FormatTime(name string, t time.Time) string {
 	case ISO8601TimeFormatName:
 		return t.Format(ISO8601OutputTimeFormat)
 	case UnixTimeFormatName:
-		return strconv.FormatInt(t.Unix(), 10)
+		ms := t.UnixNano() / int64(time.Millisecond)
+		return strconv.FormatFloat(float64(ms)/1e3, 'f', -1, 64)
 	default:
 		panic("unknown timestamp format name, " + name)
 	}

private/protocol/timestamp_test.go

@@ -16,6 +16,11 @@ func TestFormatTime(t *testing.T) {
 		"UnixTest1": {
 			formatName:     UnixTimeFormatName,
 			expectedOutput: "946845296",
+			input:          time.Date(2000, time.January, 2, 20, 34, 56, 0, time.UTC),
+		},
+		"UnixTest2": {
+			formatName:     UnixTimeFormatName,
+			expectedOutput: "946845296.123",
 			input:          time.Date(2000, time.January, 2, 20, 34, 56, .123e9, time.UTC),
 		},
 		"ISO8601Test1": {
@@ -32,8 +37,8 @@ func TestFormatTime(t *testing.T) {
 
 	for name, c := range cases {
 		t.Run(name, func(t *testing.T) {
-			if FormatTime(c.formatName, c.input) != c.expectedOutput {
-				t.Errorf("input and output time don't match for %s format ", c.formatName)
+			if e, a := c.expectedOutput, FormatTime(c.formatName, c.input); e != a {
+				t.Errorf("expected %s, got %s for %s format ", e, a, c.formatName)
 			}
 		})
 	}