diff --git a/memo/memo.go b/memo/memo.go
index 8592b17..a31c8be 100644
--- a/memo/memo.go
+++ b/memo/memo.go
@@ -1,56 +1,69 @@
 package memo
 
 import (
 	"sync"
+	"time"
 )
 
 type (
 	Memo struct {
 		f     Func
 		mu    sync.Mutex //guards cache
 		cache *entry
+
+		cacheDur  time.Duration
+		lastCache time.Time
 	}
 
 	Func func() (interface{}, error)
 )
 
 type (
 	entry struct {
 		res   result
 		ready chan struct{} // close when res is read
 	}
 
 	result struct {
 		value interface{}
 		err   error
 	}
 )
 
-func New(f Func) *Memo {
+func New(f Func, cd time.Duration) *Memo {
 	return &Memo{
-		f: f,
+		f:        f,
+		cacheDur: cd,
 	}
 }
 
-func (memo *Memo) Invalidate() {
+// Invalidate resets the cache to nil if the memo's given cache duration has
+// elapsed, and returns true if the cache was actually invalidated.
+func (memo *Memo) Invalidate() bool {
+	defer memo.mu.Unlock()
 	memo.mu.Lock()
-	memo.cache = nil
-	memo.mu.Unlock()
+
+	if memo.cache != nil && time.Now().Sub(memo.lastCache) > memo.cacheDur {
+		memo.cache = nil
+		memo.lastCache = time.Now()
+		return true
+	}
+	return false
 }
 
 func (memo *Memo) Get() (value interface{}, err error) {
 	memo.mu.Lock()
 	if memo.cache == nil {
 		memo.cache = &entry{ready: make(chan struct{})}
 		memo.mu.Unlock()
 
 		memo.cache.res.value, memo.cache.res.err = memo.f()
 
 		close(memo.cache.ready)
 	} else {
 		memo.mu.Unlock()
 
 		<-memo.cache.ready
 	}
 	return memo.cache.res.value, memo.cache.res.err
 }