expr-lang
diff --git a/docs/configuration.md b/docs/configuration.md
@@ -132,3 +132,17 @@ The timezone is used for the following functions:
 date("2024-11-23 12:00:00") // parses the date in the specified timezone
 now() // returns the current time in the specified timezone
 ```
+
+## Mock now()
+
+Expressions that depend on `now()` are hard to test because the result changes
+on every run. The [`MockNow`](https://pkg.go.dev/github.com/expr-lang/expr#MockNow)
+option replaces `now()` with a fixed time, so the compiled program always
+returns the same value.
+
+```go
+program, err := expr.Compile(code, expr.MockNow(time.Date(2024, 11, 23, 12, 0, 0, 0, time.UTC)))
+```
+
+`MockNow` can be combined with [`Timezone`](#timezone) to return the fixed time
+in the specified timezone.
diff --git a/expr.go b/expr.go
@@ -216,6 +216,16 @@ func Timezone(name string) Option {
 	})
 }
 
+// MockNow replaces now() calls with the given time. This is useful to get
+// deterministic results when testing expressions that depend on now(). The
+// replacement happens at compile time, so the compiled program always returns
+// the given time from now().
+func MockNow(t time.Time) Option {
+	return Patch(patcher.WithNow{
+		Now: t,
+	})
+}
+
 // MaxNodes sets the maximum number of nodes allowed in the expression.
 // By default, the maximum number of nodes is conf.DefaultMaxNodes.
 // If MaxNodes is set to 0, the node budget check is disabled.

diff --git a/patcher/with_now.go b/patcher/with_now.go
@@ -0,0 +1,35 @@
+package patcher
+
+import (
+	"time"
+
+	"github.com/expr-lang/expr/ast"
+)
+
+// WithNow replaces now() calls with a constant time. This is useful to get
+// deterministic results when testing expressions that depend on the current
+// time. The replacement happens while patching the AST, so the compiled
+// program always returns the same time from now().
+type WithNow struct {
+	Now time.Time
+}
+
+func (p WithNow) Visit(node *ast.Node) {
+	btin, ok := (*node).(*ast.BuiltinNode)
+	if !ok || btin.Name != "now" {
+		return
+	}
+	now := p.Now
+	// Preserve the location injected by WithTimezone, if any, so that
+	// Timezone and MockNow can be combined.
+	for _, arg := range btin.Arguments {
+		c, ok := arg.(*ast.ConstantNode)
+		if !ok {
+			continue
+		}
+		if loc, ok := c.Value.(*time.Location); ok {
+			now = now.In(loc)
+		}
+	}
+	ast.Patch(node, &ast.ConstantNode{Value: now})
+}
diff --git a/patcher/with_now_test.go b/patcher/with_now_test.go
@@ -0,0 +1,44 @@
+package patcher_test
+
+import (
+	"testing"
+	"time"
+
+	"github.com/expr-lang/expr/internal/testify/require"
+
+	"github.com/expr-lang/expr"
+)
+
+func TestWithNow(t *testing.T) {
+	now := time.Date(2024, 5, 7, 23, 0, 0, 0, time.UTC)
+
+	program, err := expr.Compile(`now()`, expr.MockNow(now))
+	require.NoError(t, err)
+
+	out, err := expr.Run(program, nil)
+	require.NoError(t, err)
+	require.Equal(t, now, out)
+}
+
+func TestWithNow_is_deterministic(t *testing.T) {
+	now := time.Date(2024, 5, 7, 23, 0, 0, 0, time.UTC)
+
+	program, err := expr.Compile(`now().Year()`, expr.MockNow(now))
+	require.NoError(t, err)
+
+	out, err := expr.Run(program, nil)
+	require.NoError(t, err)
+	require.Equal(t, 2024, out)
+}
+
+func TestWithNow_combined_with_timezone(t *testing.T) {
+	now := time.Date(2024, 5, 7, 23, 0, 0, 0, time.UTC)
+
+	program, err := expr.Compile(`now()`, expr.Timezone("Asia/Kamchatka"), expr.MockNow(now))
+	require.NoError(t, err)
+
+	out, err := expr.Run(program, nil)
+	require.NoError(t, err)
+	require.Equal(t, "Asia/Kamchatka", out.(time.Time).Location().String())
+	require.True(t, out.(time.Time).Equal(now))
+}