diff --git a/Makefile b/Makefile
index 954864a..c5dc744 100644
--- a/Makefile
+++ b/Makefile
@@ -16,3 +16,7 @@ generate: mod-tidy
 build:
 	GOVERSION=$$(go version) \
 						goreleaser build --clean --debug --single-target --snapshot
+
+.PHONY: fuzz
+fuzz: mod-tidy generate
+	go test -fuzz='^Fuzz' -fuzztime=10s -v ./internal/server
diff --git a/cmd/go-cli-github/main.go b/cmd/go-cli-github/main.go
index 2cc9a5c..e98707f 100644
--- a/cmd/go-cli-github/main.go
+++ b/cmd/go-cli-github/main.go
@@ -1,3 +1,4 @@
+// Package main implements the command-line interface of a server.
 package main
 
 import (
diff --git a/cmd/go-cli-github/serve.go b/cmd/go-cli-github/serve.go
index a80e5f0..fe7016c 100644
--- a/cmd/go-cli-github/serve.go
+++ b/cmd/go-cli-github/serve.go
@@ -2,6 +2,7 @@ package main
 
 import (
 	"fmt"
+	"time"
 
 	"github.com/smlx/go-cli-github/internal/server"
 )
@@ -11,6 +12,6 @@ type ServeCmd struct{}
 
 // Run the serve command.
 func (*ServeCmd) Run() error {
-	fmt.Println(server.Serve())
+	fmt.Println(server.New(time.Now).Serve())
 	return nil
 }
diff --git a/go.mod b/go.mod
index c90281c..cc14af5 100644
--- a/go.mod
+++ b/go.mod
@@ -2,4 +2,12 @@ module github.com/smlx/go-cli-github
 
 go 1.21
 
-require github.com/alecthomas/kong v0.8.1
+require (
+	github.com/alecthomas/assert/v2 v2.1.0
+	github.com/alecthomas/kong v0.8.1
+)
+
+require (
+	github.com/alecthomas/repr v0.1.0 // indirect
+	github.com/hexops/gotextdiff v1.0.3 // indirect
+)
diff --git a/internal/server/serve.go b/internal/server/serve.go
index d89e008..1113065 100644
--- a/internal/server/serve.go
+++ b/internal/server/serve.go
@@ -1,6 +1,55 @@
+// Package server implements an example server.
 package server
 
+import (
+	"fmt"
+	"time"
+)
+
+// Server is an example server.
+type Server struct {
+	// now is a function returning a time that the server will consider the
+	// current instant.
+	now func() time.Time
+}
+
+// New constructs a new Server, which greets people based on the time returned
+// by the given nowFunc. If nowFunc is nil, the Server will default to time.Now.
+func New(nowFunc func() time.Time) *Server {
+	return &Server{
+		now: nowFunc,
+	}
+}
+
 // Serve is an example function.
-func Serve() string {
+func (*Server) Serve() string {
 	return "example serve command"
 }
+
+// Greet is an example fuzzable function.
+// name is free form, but birthDate must be in YYYY-MM-DD format.
+func (s *Server) Greet(name, birthDate string) (string, error) {
+	b, err := time.ParseInLocation("2006-01-02", birthDate, time.Local)
+	if err != nil {
+		return "", fmt.Errorf("couldn't parse birthDate: %v", err)
+	}
+	now := s.now()
+	if now.Before(b) {
+		return "", fmt.Errorf("time travel detected")
+	}
+	// calculate time since birthday this year
+	d := now.Sub(
+		time.Date(now.Year(), b.Month(), b.Day(), 0, 0, 0, 0, time.Local))
+	switch {
+	case d < 0:
+		// calculate time since birthday last year
+		d = now.Sub(
+			time.Date(now.Year()-1, b.Month(), b.Day(), 0, 0, 0, 0, time.Local))
+		fallthrough
+	case d > 0:
+		return fmt.Sprintf("Hello %s, happy belated birthday for %d days ago.",
+			name, int(d.Hours()/24)), nil
+	default:
+		return fmt.Sprintf("Hello %s, happy birthday!", name), nil
+	}
+}
diff --git a/internal/server/serve_test.go b/internal/server/serve_test.go
index 68cedc3..7de175f 100644
--- a/internal/server/serve_test.go
+++ b/internal/server/serve_test.go
@@ -2,10 +2,20 @@ package server_test
 
 import (
 	"testing"
+	"time"
 
+	"github.com/alecthomas/assert/v2"
 	"github.com/smlx/go-cli-github/internal/server"
 )
 
+func nowTestFunc() time.Time {
+	t, err := time.ParseInLocation("2006-01-02", "2023-12-12", time.Local)
+	if err != nil {
+		panic(err)
+	}
+	return t
+}
+
 func TestServe(t *testing.T) {
 	var testCases = map[string]struct {
 		input  string
@@ -15,10 +25,50 @@ func TestServe(t *testing.T) {
 	}
 	for name, tc := range testCases {
 		t.Run(name, func(tt *testing.T) {
-			result := server.Serve()
-			if result != tc.expect {
-				tt.Fatalf("expected %s, got %s", tc.expect, result)
+			s := server.New(nowTestFunc)
+			result := s.Serve()
+			assert.Equal(tt, tc.expect, result, name)
+		})
+	}
+}
+
+func TestGreet(t *testing.T) {
+	var testCases = map[string]struct {
+		input       []string
+		expect      string
+		expectError bool
+	}{
+		"boomer": {
+			input:  []string{"Jim", "1963-02-03"},
+			expect: "Hello Jim, happy belated birthday for 312 days ago.",
+		},
+		"the doctor": {
+			input:       []string{"Who", "2963-02-03"},
+			expect:      "time travel detected",
+			expectError: true,
+		},
+	}
+	for name, tc := range testCases {
+		t.Run(name, func(tt *testing.T) {
+			s := server.New(nowTestFunc)
+			result, err := s.Greet(tc.input[0], tc.input[1])
+			if tc.expectError {
+				assert.EqualError(tt, err, tc.expect, name)
+			} else {
+				assert.NoError(tt, err, name)
+				assert.Equal(tt, tc.expect, result, name)
 			}
 		})
 	}
 }
+
+func FuzzGreet(f *testing.F) {
+	f.Add("Joe", "2020-04-02")
+	f.Fuzz(func(t *testing.T, name, birthDate string) {
+		s := server.New(nowTestFunc)
+		out, err := s.Greet(name, birthDate)
+		if err != nil && out != "" {
+			t.Errorf("%q, %v", out, err)
+		}
+	})
+}