diff --git a/cmd/cerbos/repl/internal/repl.go b/cmd/cerbos/repl/internal/repl.go
index 7f2b93ea4..2dd04abab 100644
--- a/cmd/cerbos/repl/internal/repl.go
+++ b/cmd/cerbos/repl/internal/repl.go
@@ -17,7 +17,6 @@ import (
 	"sort"
 	"strconv"
 	"strings"
-	"time"
 	"unicode"
 
 	"github.com/alecthomas/participle/v2"
@@ -411,7 +410,7 @@ func (r *REPL) evalExpr(expr string) (ref.Val, *exprpb.Type, error) {
 		return nil, nil, errSilent
 	}
 
-	val, _, err := conditions.Eval(env, ast, r.vars, time.Now)
+	val, _, err := conditions.Eval(env, ast, r.vars, conditions.Now())
 	if err != nil {
 		return nil, nil, err
 	}
diff --git a/cmd/cerbosctl/put/put_test.go b/cmd/cerbosctl/put/put_test.go
index e20e1766c..fd0da72ae 100644
--- a/cmd/cerbosctl/put/put_test.go
+++ b/cmd/cerbosctl/put/put_test.go
@@ -111,6 +111,7 @@ func testPutCmd(clientCtx *cmdclient.Context, globals *flagset.Globals) func(*te
 					"resource.leave_request.vdefault/acme.hr.uk",
 					"resource.leave_request.vstaging",
 					"resource.missing_attr.vdefault",
+					"resource.output_now.vdefault",
 					"resource.products.vdefault",
 					"resource.purchase_order.vdefault",
 					"resource.runtime_effective_derived_roles.vdefault",
diff --git a/internal/conditions/cerbos_lib.go b/internal/conditions/cerbos_lib.go
index d771e9586..ba4a6e32f 100644
--- a/internal/conditions/cerbos_lib.go
+++ b/internal/conditions/cerbos_lib.go
@@ -127,11 +127,21 @@ func (clib cerbosLib) ProgramOptions() []cel.ProgramOption {
 	return nil
 }
 
+type NowFunc = func() time.Time
+
+// Now returns a NowFunc that always returns the time at which Now was called.
+func Now() NowFunc {
+	now := time.Now()
+	return func() time.Time { return now }
+}
+
 // Eval returns the result of an evaluation of the ast and environment against the input vars,
 // providing time-based functions with a static definition of the current time.
 //
+// The given nowFunc must return the same timestamp each time it is called.
+//
 // See https://pkg.go.dev/github.com/google/cel-go/cel#Program.Eval.
-func Eval(env *cel.Env, ast *cel.Ast, vars any, nowFunc func() time.Time, opts ...cel.ProgramOption) (ref.Val, *cel.EvalDetails, error) {
+func Eval(env *cel.Env, ast *cel.Ast, vars any, nowFunc NowFunc, opts ...cel.ProgramOption) (ref.Val, *cel.EvalDetails, error) {
 	programOpts := append([]cel.ProgramOption{cel.CustomDecorator(newTimeDecorator(nowFunc))}, opts...)
 	prg, err := env.Program(ast, programOpts...)
 	if err != nil {
@@ -141,13 +151,13 @@ func Eval(env *cel.Env, ast *cel.Ast, vars any, nowFunc func() time.Time, opts .
 	return prg.Eval(vars)
 }
 
-func newTimeDecorator(nowFunc func() time.Time) interpreter.InterpretableDecorator {
-	td := timeDecorator{now: nowFunc()}
+func newTimeDecorator(nowFunc NowFunc) interpreter.InterpretableDecorator {
+	td := timeDecorator{nowFunc: nowFunc}
 	return td.decorate
 }
 
 type timeDecorator struct {
-	now time.Time
+	nowFunc NowFunc
 }
 
 func (t *timeDecorator) decorate(in interpreter.Interpretable) (interpreter.Interpretable, error) {
@@ -159,7 +169,7 @@ func (t *timeDecorator) decorate(in interpreter.Interpretable) (interpreter.Inte
 	funcName := call.Function()
 	switch funcName {
 	case nowFn:
-		return interpreter.NewConstValue(call.ID(), types.DefaultTypeAdapter.NativeToValue(t.now)), nil
+		return interpreter.NewConstValue(call.ID(), types.DefaultTypeAdapter.NativeToValue(t.nowFunc())), nil
 	case timeSinceFn:
 		return interpreter.NewCall(call.ID(), funcName, call.OverloadID(), call.Args(), func(values ...ref.Val) ref.Val {
 			if len(values) != 1 {
@@ -172,7 +182,7 @@ func (t *timeDecorator) decorate(in interpreter.Interpretable) (interpreter.Inte
 				return types.NoSuchOverloadErr()
 			}
 
-			return types.DefaultTypeAdapter.NativeToValue(t.now.Sub(ts))
+			return types.DefaultTypeAdapter.NativeToValue(t.nowFunc().Sub(ts))
 		}), nil
 	default:
 		return in, nil
diff --git a/internal/conditions/cerbos_lib_test.go b/internal/conditions/cerbos_lib_test.go
index d5468af0c..d0b004a45 100644
--- a/internal/conditions/cerbos_lib_test.go
+++ b/internal/conditions/cerbos_lib_test.go
@@ -106,7 +106,7 @@ func TestCerbosLib(t *testing.T) {
 			ast, issues := env.Compile(tc.expr)
 			is.NoError(issues.Err())
 
-			have, _, err := conditions.Eval(env, ast, cel.NoVars(), time.Now)
+			have, _, err := conditions.Eval(env, ast, cel.NoVars(), conditions.Now())
 			if tc.wantErr {
 				is.Error(err)
 			} else {
diff --git a/internal/engine/engine.go b/internal/engine/engine.go
index 6030fbc33..61810871c 100644
--- a/internal/engine/engine.go
+++ b/internal/engine/engine.go
@@ -24,6 +24,7 @@ import (
 	runtimev1 "github.com/cerbos/cerbos/api/genpb/cerbos/runtime/v1"
 	schemav1 "github.com/cerbos/cerbos/api/genpb/cerbos/schema/v1"
 	"github.com/cerbos/cerbos/internal/audit"
+	"github.com/cerbos/cerbos/internal/conditions"
 	"github.com/cerbos/cerbos/internal/engine/planner"
 	"github.com/cerbos/cerbos/internal/engine/tracer"
 	"github.com/cerbos/cerbos/internal/namer"
@@ -87,6 +88,10 @@ func newCheckOptions(ctx context.Context, conf *Conf, opts ...CheckOpt) *CheckOp
 		opt(co)
 	}
 
+	if co.evalParams.nowFunc == nil {
+		co.evalParams.nowFunc = conditions.Now()
+	}
+
 	return co
 }
 
@@ -105,6 +110,7 @@ func WithZapTraceSink(log *zap.Logger) CheckOpt {
 }
 
 // WithNowFunc sets the function for determining `now` during condition evaluation.
+// The function should return the same timestamp every time it is invoked.
 func WithNowFunc(nowFunc func() time.Time) CheckOpt {
 	return func(co *CheckOptions) {
 		co.evalParams.nowFunc = nowFunc
@@ -262,10 +268,8 @@ func (engine *Engine) doPlanResources(ctx context.Context, input *enginev1.PlanR
 
 	result := new(planner.PolicyPlanResult)
 	auditTrail := &auditv1.AuditTrail{EffectivePolicies: make(map[string]*policyv1.SourceAttributes, 2)} //nolint:mnd
-	now := opts.NowFunc()()
-	nowFn := func() time.Time { return now }
 	if policy := policySet.GetPrincipalPolicy(); policy != nil {
-		policyEvaluator := planner.PrincipalPolicyEvaluator{Policy: policy, Globals: opts.Globals(), NowFn: nowFn}
+		policyEvaluator := planner.PrincipalPolicyEvaluator{Policy: policy, Globals: opts.Globals(), NowFn: opts.NowFunc()}
 		result, err = policyEvaluator.EvaluateResourcesQueryPlan(ctx, input)
 		if err != nil {
 			return nil, nil, err
@@ -282,7 +286,7 @@ func (engine *Engine) doPlanResources(ctx context.Context, input *enginev1.PlanR
 	}
 
 	if policy := policySet.GetResourcePolicy(); policy != nil {
-		policyEvaluator := planner.ResourcePolicyEvaluator{Policy: policy, Globals: opts.Globals(), SchemaMgr: engine.schemaMgr, NowFn: nowFn}
+		policyEvaluator := planner.ResourcePolicyEvaluator{Policy: policy, Globals: opts.Globals(), SchemaMgr: engine.schemaMgr, NowFn: opts.NowFunc()}
 		plan, err := policyEvaluator.EvaluateResourcesQueryPlan(ctx, input)
 		if err != nil {
 			return nil, nil, err
diff --git a/internal/engine/engine_test.go b/internal/engine/engine_test.go
index 2176d67c1..74935ca61 100644
--- a/internal/engine/engine_test.go
+++ b/internal/engine/engine_test.go
@@ -76,6 +76,47 @@ func TestCheck(t *testing.T) {
 			))
 		})
 	}
+
+	t.Run("deterministic_now", func(t *testing.T) {
+		roles := []string{"user"}
+		actions := []string{"a", "b", "c"}
+
+		inputs := []*enginev1.CheckInput{
+			{
+				Principal: &enginev1.Principal{Id: "1", Roles: roles},
+				Resource:  &enginev1.Resource{Kind: "output_now", Id: "1"},
+				Actions:   actions,
+			},
+			{
+				Principal: &enginev1.Principal{Id: "2", Roles: roles},
+				Resource:  &enginev1.Resource{Kind: "output_now", Id: "1"},
+				Actions:   actions,
+			},
+			{
+				Principal: &enginev1.Principal{Id: "1", Roles: roles},
+				Resource:  &enginev1.Resource{Kind: "output_now", Id: "2"},
+				Actions:   actions,
+			},
+			{
+				Principal: &enginev1.Principal{Id: "2", Roles: roles},
+				Resource:  &enginev1.Resource{Kind: "output_now", Id: "2"},
+				Actions:   actions,
+			},
+		}
+
+		outputs, err := eng.Check(context.Background(), inputs)
+		require.NoError(t, err)
+		require.Len(t, outputs, len(inputs))
+
+		uniqueNows := make(map[string]struct{})
+		for _, output := range outputs {
+			require.Len(t, output.Outputs, 3)
+			for _, entry := range output.Outputs {
+				uniqueNows[entry.Val.GetStringValue()] = struct{}{}
+			}
+		}
+		require.Len(t, uniqueNows, 1)
+	})
 }
 
 func TestCheckWithLenientScopeSearch(t *testing.T) {
@@ -306,19 +347,13 @@ func TestQueryPlan(t *testing.T) {
 						IncludeMeta: true,
 						AuxData:     auxData,
 					}
-					nowFnCallsCounter := 0
-					nowFn := func() time.Time {
-						nowFnCallsCounter++
-						return timestamp
-					}
-					response, err := eng.PlanResources(context.Background(), request, WithNowFunc(nowFn))
+					response, err := eng.PlanResources(context.Background(), request, WithNowFunc(func() time.Time { return timestamp }))
 					if tt.WantErr {
 						is.Error(err)
 					} else {
 						is.NoError(err)
 						is.NotNil(response)
 						is.Empty(cmp.Diff(tt.Want, response.Filter, protocmp.Transform()), "AST: %s\n%s\n", response.FilterDebug, protojson.Format(response.Filter))
-						is.Equal(1, nowFnCallsCounter, "time function should be called once")
 					}
 				})
 			}
diff --git a/internal/engine/evaluator.go b/internal/engine/evaluator.go
index 11868d372..099b9e7ee 100644
--- a/internal/engine/evaluator.go
+++ b/internal/engine/evaluator.go
@@ -9,7 +9,6 @@ import (
 	"fmt"
 	"reflect"
 	"sort"
-	"time"
 
 	"github.com/google/cel-go/cel"
 	"github.com/google/cel-go/common/types"
@@ -41,7 +40,7 @@ var ErrPolicyNotExecutable = errors.New("policy not executable")
 
 type evalParams struct {
 	globals              map[string]any
-	nowFunc              func() time.Time
+	nowFunc              conditions.NowFunc
 	defaultPolicyVersion string
 	lenientScopeSearch   bool
 }
@@ -49,7 +48,6 @@ type evalParams struct {
 func defaultEvalParams(conf *Conf) evalParams {
 	return evalParams{
 		globals:              conf.Globals,
-		nowFunc:              time.Now,
 		defaultPolicyVersion: conf.DefaultPolicyVersion,
 		lenientScopeSearch:   conf.LenientScopeSearch,
 	}
diff --git a/internal/storage/index/builder_test.go b/internal/storage/index/builder_test.go
index 21ba85d94..5b381bcb6 100644
--- a/internal/storage/index/builder_test.go
+++ b/internal/storage/index/builder_test.go
@@ -45,7 +45,7 @@ func TestBuildIndexWithDisk(t *testing.T) {
 
 	t.Run("check_contents", func(t *testing.T) {
 		data := idxImpl.Inspect()
-		require.Len(t, data, 38)
+		require.Len(t, data, 39)
 
 		rp1 := filepath.Join("resource_policies", "policy_01.yaml")
 		rp2 := filepath.Join("resource_policies", "policy_02.yaml")
diff --git a/internal/test/testdata/store/resource_policies/policy_15.yaml b/internal/test/testdata/store/resource_policies/policy_15.yaml
new file mode 100644
index 000000000..52577566a
--- /dev/null
+++ b/internal/test/testdata/store/resource_policies/policy_15.yaml
@@ -0,0 +1,36 @@
+# yaml-language-server: $schema=../../../../../schema/jsonschema/cerbos/policy/v1/Policy.schema.json
+---
+apiVersion: api.cerbos.dev/v1
+resourcePolicy:
+  version: default
+  resource: output_now
+  rules:
+    - name: a
+      actions:
+        - a
+      effect: EFFECT_ALLOW
+      roles:
+        - user
+      output:
+        when:
+          ruleActivated: now()
+
+    - name: b
+      actions:
+        - b
+      effect: EFFECT_ALLOW
+      roles:
+        - user
+      output:
+        when:
+          ruleActivated: now()
+
+    - name: c
+      actions:
+        - c
+      effect: EFFECT_ALLOW
+      roles:
+        - user
+      output:
+        when:
+          ruleActivated: now()
diff --git a/private/verify/verify.go b/private/verify/verify.go
index ebcfc43d1..3e6e1d85a 100644
--- a/private/verify/verify.go
+++ b/private/verify/verify.go
@@ -94,6 +94,7 @@ func Bundle(ctx context.Context, params BundleParams) (*policyv1.TestResults, er
 type CheckOptions interface {
 	Globals() map[string]any
 	NowFunc() func() time.Time
+	DefaultPolicyVersion() string
 	LenientScopeSearch() bool
 }