diff --git a/engine.go b/engine.go
index b1db42e..ba90a4f 100644
--- a/engine.go
+++ b/engine.go
@@ -9,6 +9,7 @@ import (
 	"io"
 
 	"github.com/osteele/liquid/chunks"
+	"github.com/osteele/liquid/expressions"
 	"github.com/osteele/liquid/filters"
 	"github.com/osteele/liquid/tags"
 )
@@ -23,6 +24,7 @@ func init() {
 //
 // In the future, it will be configured with additional tags, filters, and the {%include%} search path.
 type Engine interface {
+	DefineFilter(name string, fn interface{})
 	DefineTag(string, func(form string) (func(io.Writer, chunks.Context) error, error))
 
 	ParseTemplate(text []byte) (Template, error)
@@ -43,7 +45,13 @@ func NewEngine() Engine {
 	return engine{}
 }
 
+func (e engine) DefineFilter(name string, fn interface{}) {
+	// TODO define this on the engine, not globally
+	expressions.DefineFilter(name, fn)
+}
+
 func (e engine) DefineTag(name string, td func(form string) (func(io.Writer, chunks.Context) error, error)) {
+	// TODO define this on the engine, not globally
 	chunks.DefineTag(name, chunks.TagDefinition(td))
 }
 
diff --git a/expressions/filters.go b/expressions/filters.go
index a9a660b..08641e0 100644
--- a/expressions/filters.go
+++ b/expressions/filters.go
@@ -3,6 +3,7 @@ package expressions
 import (
 	"fmt"
 	"reflect"
+	"runtime/debug"
 
 	"github.com/osteele/liquid/errors"
 	"github.com/osteele/liquid/generics"
@@ -48,7 +49,7 @@ func makeFilter(f valueFn, name string, param valueFn) valueFn {
 				case generics.GenericError:
 					panic(InterpreterError(e.Error()))
 				default:
-					// fmt.Println(string(debug.Stack()))
+					fmt.Println(string(debug.Stack()))
 					panic(e)
 				}
 			}
@@ -57,7 +58,7 @@ func makeFilter(f valueFn, name string, param valueFn) valueFn {
 		if param != nil {
 			args = append(args, param(ctx))
 		}
-		out, err := generics.Apply(fr, args)
+		out, err := generics.Call(fr, args)
 		if err != nil {
 			panic(err)
 		}
diff --git a/filters/filter_test.go b/filters/filter_test.go
index 9a7d0a4..b0454f3 100644
--- a/filters/filter_test.go
+++ b/filters/filter_test.go
@@ -3,6 +3,7 @@ package filters
 import (
 	"fmt"
 	"testing"
+	"time"
 
 	"github.com/osteele/liquid/expressions"
 	"github.com/stretchr/testify/require"
@@ -13,11 +14,18 @@ func init() {
 }
 
 var filterTests = []struct{ in, expected string }{
-	// Jekyll extensions
-	{`obj | inspect`, `{"a":1}`},
+	// values
+	{`4.99 | default: 2.99`, "4.99"},
+	{`undefined | default: 2.99`, "2.99"},
+	{`false | default: 2.99`, "2.99"},
+	{`empty_list | default: 2.99`, "2.99"},
 
-	// filters
-	// product_price | default: 2.99 }}
+	// date filters
+	{`article.published_at | date`, "Fri, Jul 17, 15"},
+	// article.published_at | date: "%a, %b %d, %y"
+	// article.published_at | date: "%Y"
+	// "March 14, 2016" | date: "%b %d, %y"
+	// "now" | date: "%Y-%m-%d %H:%M" }
 
 	// list filters
 	// site.pages | map: 'category' | compact | join "," %}
@@ -64,19 +72,28 @@ var filterTests = []struct{ in, expected string }{
 	// 183.357 | floor
 	// minus, modulo, plus, round,times
 
-	// date filters
-	// article.published_at | date: "%a, %b %d, %y"
-	// article.published_at | date: "%Y"
-	// "March 14, 2016" | date: "%b %d, %y"
-	// "now" | date: "%Y-%m-%d %H:%M" }
+	// Jekyll extensions
+	{`obj | inspect`, `{"a":1}`},
+}
+
+func timeMustParse(s string) time.Time {
+	t, err := time.Parse(time.RFC3339, s)
+	if err != nil {
+		panic(err)
+	}
+	return t
 }
 
 var filterTestContext = expressions.NewContext(map[string]interface{}{
-	"x": 123,
+	"x":          123,
+	"empty_list": map[string]interface{}{},
 	"obj": map[string]interface{}{
 		"a": 1,
 	},
 	"animals": []string{"zebra", "octopus", "giraffe", "Sally Snake"},
+	"article": map[string]interface{}{
+		"published_at": timeMustParse("2015-07-17T15:04:05Z"),
+	},
 	"pages": []map[string]interface{}{
 		{"category": "business"},
 		{"category": "celebrities"},
@@ -103,7 +120,7 @@ func TestFilters(t *testing.T) {
 		t.Run(fmt.Sprintf("%02d", i), func(t *testing.T) {
 			val, err := expressions.EvaluateExpr(test.in, filterTestContext)
 			require.NoErrorf(t, err, test.in)
-			actual := fmt.Sprintf("%s", val)
+			actual := fmt.Sprintf("%v", val)
 			require.Equalf(t, test.expected, actual, test.in)
 		})
 	}
diff --git a/filters/filters.go b/filters/filters.go
index af2b8e9..7191302 100644
--- a/filters/filters.go
+++ b/filters/filters.go
@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"strings"
+	"time"
 
 	"github.com/osteele/liquid/expressions"
 	"github.com/osteele/liquid/generics"
@@ -12,6 +13,27 @@ import (
 
 // DefineStandardFilters defines the standard Liquid filters.
 func DefineStandardFilters() {
+	// values
+	expressions.DefineFilter("default", func(in, defaultValue interface{}) interface{} {
+		if in == nil || in == false || generics.IsEmpty(in) {
+			in = defaultValue
+		}
+		return in
+	})
+
+	// dates
+	expressions.DefineFilter("date", func(in, format interface{}) interface{} {
+		if format != nil {
+			panic("date conversion format is not implemented")
+		}
+		switch in := in.(type) {
+		case time.Time:
+			return in.Format("Mon, Jan 2, 06")
+		default:
+			panic("unimplemented date conversion")
+		}
+	})
+
 	// lists
 	expressions.DefineFilter("join", joinFilter)
 	expressions.DefineFilter("sort", sortFilter)
diff --git a/generics/call.go b/generics/call.go
new file mode 100644
index 0000000..bd371ca
--- /dev/null
+++ b/generics/call.go
@@ -0,0 +1,44 @@
+package generics
+
+import (
+	"fmt"
+	"reflect"
+)
+
+// Call applies a function to arguments, converting them as necessary.
+// The conversion follows Liquid semantics, which are more aggressive than
+// Go conversion. The function should return one or two values; the second value,
+// if present, should be an error.
+func Call(fn reflect.Value, args []interface{}) (interface{}, error) {
+	in := convertArguments(fn, args)
+	outs := fn.Call(in)
+	if len(outs) > 1 && outs[1].Interface() != nil {
+		switch e := outs[1].Interface().(type) {
+		case error:
+			fmt.Println("error")
+			return nil, e
+		default:
+			panic(e)
+		}
+	}
+	return outs[0].Interface(), nil
+}
+
+// Convert args to match the input types of function fn.
+func convertArguments(fn reflect.Value, in []interface{}) []reflect.Value {
+	rt := fn.Type()
+	out := make([]reflect.Value, rt.NumIn())
+	for i, arg := range in {
+		if i < rt.NumIn() {
+			if arg == nil {
+				out[i] = reflect.Zero(rt.In(i))
+			} else {
+				out[i] = convertType(arg, rt.In(i))
+			}
+		}
+	}
+	for i := len(in); i < rt.NumIn(); i++ {
+		out[i] = reflect.Zero(rt.In(i))
+	}
+	return out
+}
diff --git a/generics/generics.go b/generics/generics.go
index dbd5f91..04dbd31 100644
--- a/generics/generics.go
+++ b/generics/generics.go
@@ -14,24 +14,6 @@ func genericErrorf(format string, a ...interface{}) error {
 	return GenericError(fmt.Sprintf(format, a...))
 }
 
-// Apply applies a function to arguments, converting them as necessary.
-// The conversion follows Liquid semantics, which are more aggressive than
-// Go conversion. The function should return one or two values; the second value,
-// if present, should be an error.
-func Apply(fn reflect.Value, args []interface{}) (interface{}, error) {
-	in := convertArguments(fn, args)
-	outs := fn.Call(in)
-	if len(outs) > 1 && outs[1].Interface() != nil {
-		switch e := outs[1].Interface().(type) {
-		case error:
-			return nil, e
-		default:
-			panic(e)
-		}
-	}
-	return outs[0].Interface(), nil
-}
-
 // Convert val to the type. This is a more aggressive conversion, that will
 // recursively create new map and slice values as necessary. It doesn't
 // handle circular references.
@@ -58,17 +40,18 @@ func convertType(val interface{}, t reflect.Type) reflect.Value {
 	panic(genericErrorf("convertType: can't convert %#v<%s> to %v", val, r.Type(), t))
 }
 
-// Convert args to match the input types of function fn.
-func convertArguments(fn reflect.Value, in []interface{}) []reflect.Value {
-	rt := fn.Type()
-	out := make([]reflect.Value, rt.NumIn())
-	for i, arg := range in {
-		if i < rt.NumIn() {
-			out[i] = convertType(arg, rt.In(i))
-		}
+// IsEmpty returns a bool indicating whether the value is empty according to Liquid semantics.
+func IsEmpty(in interface{}) bool {
+	if in == nil {
+		return false
 	}
-	for i := len(in); i < rt.NumIn(); i++ {
-		out[i] = reflect.Zero(rt.In(i))
+	r := reflect.ValueOf(in)
+	switch r.Kind() {
+	case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
+		return r.Len() == 0
+	case reflect.Bool:
+		return r.Bool() == false
+	default:
+		return false
 	}
-	return out
 }