Adds IsZero functionality to DateTime

This PR adds a new function `IsZero` to the DateTime struct. The goal of
this is to satisfy a common interface, IsZeroer, used by go-yaml/yaml
marshaller. `IsZero` already exists on the time.Time object, so this
does not introduce any novel implementation.

For reference, here is the [documentation
note](https://pkg.go.dev/gopkg.in/yaml.v3#Marshal) on using IsZero by the
`(yaml).Marshal` function.

Signed-off-by: Brandon Gonzalez <bg@chronosphere.io>

time.go

@@ -123,6 +123,11 @@ func (t DateTime) String() string {
 	return NormalizeTimeForMarshal(time.Time(t)).Format(MarshalFormat)
 }
 
+// IsZero returns whether the date time is a zero value
+func (t DateTime) IsZero() bool {
+	return time.Time(t).IsZero()
+}
+
 // MarshalText implements the text marshaller interface
 func (t DateTime) MarshalText() ([]byte, error) {
 	return []byte(t.String()), nil

time_test.go

@@ -53,6 +53,16 @@ func TestNewDateTime(t *testing.T) {
 	assert.EqualValues(t, time.Unix(0, 0).UTC(), NewDateTime())
 }
 
+func TestIsZero(t *testing.T) {
+	var empty DateTime
+	assert.True(t, empty.IsZero())
+	assert.False(t, DateTime(time.Unix(100, 5)).IsZero())
+
+	// time.Unix(0,0) does not produce a true zero value struct,
+	// so this is expected to fail.
+	assert.False(t, NewDateTime().IsZero())
+}
+
 func TestParseDateTime_errorCases(t *testing.T) {
 	_, err := ParseDateTime("yada")
 	assert.Error(t, err)