diff --git a/internal/lang/funcs/datetime.go b/internal/lang/funcs/datetime.go
index 9907428ac232..a8e5ae6872bd 100644
--- a/internal/lang/funcs/datetime.go
+++ b/internal/lang/funcs/datetime.go
@@ -28,6 +28,9 @@ func MakeStaticTimestampFunc(static time.Time) function.Function {
 		Params: []function.Parameter{},
 		Type:   function.StaticReturnType(cty.String),
 		Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
+			if static.IsZero() {
+				return cty.UnknownVal(cty.String), nil
+			}
 			return cty.StringVal(static.Format(time.RFC3339)), nil
 		},
 	})
diff --git a/internal/lang/functions_test.go b/internal/lang/functions_test.go
index 33df86f464f3..7ccbd1afb5f3 100644
--- a/internal/lang/functions_test.go
+++ b/internal/lang/functions_test.go
@@ -1377,6 +1377,26 @@ func TestFunctions(t *testing.T) {
 	}
 }
 
+func TestPlanTimeStampUnknown(t *testing.T) {
+	// plantimestamp should return an unknown if there is no timestamp, which
+	// happens during validation
+	expr, parseDiags := hclsyntax.ParseExpression([]byte("plantimestamp()"), "test.hcl", hcl.Pos{Line: 1, Column: 1})
+	if parseDiags.HasErrors() {
+		t.Fatal(parseDiags)
+	}
+
+	scope := &Scope{}
+	got, diags := scope.EvalExpr(expr, cty.DynamicPseudoType)
+	if diags.HasErrors() {
+		t.Fatal(diags.Err())
+
+	}
+
+	if got.IsKnown() {
+		t.Fatalf("plantimestamp() should be unknown, got %#v\n", got)
+	}
+}
+
 const (
 	CipherBase64 = "eczGaDhXDbOFRZGhjx2etVzWbRqWDlmq0bvNt284JHVbwCgObiuyX9uV0LSAMY707IEgMkExJqXmsB4OWKxvB7epRB9G/3+F+pcrQpODlDuL9oDUAsa65zEpYF0Wbn7Oh7nrMQncyUPpyr9WUlALl0gRWytOA23S+y5joa4M34KFpawFgoqTu/2EEH4Xl1zo+0fy73fEto+nfkUY+meuyGZ1nUx/+DljP7ZqxHBFSlLODmtuTMdswUbHbXbWneW51D7Jm7xB8nSdiA2JQNK5+Sg5x8aNfgvFTt/m2w2+qpsyFa5Wjeu6fZmXSl840CA07aXbk9vN4I81WmJyblD/ZA=="
 	PrivateKey   = `