Parse input timestamps with standard RFC3339

Fix for #13175.

This change allows user-input timestamps (e.g. to `docker events
--since/--until`  or `docker logs --since` to be parsed using
standard RFC3339Nano layout in Go instead of the layout that parses
all timestamps into fixed-length strings (currently buggy).

User inputs need not to be complying to the internal format
(`RFC3339NanoFixed`) anyway.

Added test case for `events --since/--until` with all possible
timestamp input formats.

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>

timeutils/utils_test.go

@@ -0,0 +1,36 @@
+package timeutils
+
+import (
+	"testing"
+)
+
+func TestGetTimestamp(t *testing.T) {
+	cases := []struct{ in, expected string }{
+		{"0", "-62167305600"}, // 0 gets parsed year 0
+
+		// Partial RFC3339 strings get parsed with second precision
+		{"2006-01-02T15:04:05.999999999+07:00", "1136189045"},
+		{"2006-01-02T15:04:05.999999999Z", "1136214245"},
+		{"2006-01-02T15:04:05.999999999", "1136214245"},
+		{"2006-01-02T15:04:05", "1136214245"},
+		{"2006-01-02T15:04", "1136214240"},
+		{"2006-01-02T15", "1136214000"},
+		{"2006-01-02T", "1136160000"},
+		{"2006-01-02", "1136160000"},
+		{"2006", "1136073600"},
+		{"2015-05-13T20:39:09Z", "1431549549"},
+
+		// unix timestamps returned as is
+		{"1136073600", "1136073600"},
+
+		// String fallback
+		{"invalid", "invalid"},
+	}
+
+	for _, c := range cases {
+		o := GetTimestamp(c.in)
+		if o != c.expected {
+			t.Fatalf("wrong value for '%s'. expected:'%s' got:'%s'", c.in, c.expected, o)
+		}
+	}
+}