diff --git a/src/main/java/hse/java/lectures/lesson7/dau/DauServiceImpl.java b/src/main/java/hse/java/lectures/lesson7/dau/DauServiceImpl.java
new file mode 100644
index 0000000..ad86d43
--- /dev/null
+++ b/src/main/java/hse/java/lectures/lesson7/dau/DauServiceImpl.java
@@ -0,0 +1,51 @@
+package hse.java.lectures.lesson7.dau;
+
+import java.time.Clock;
+import java.time.LocalDate;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+
+public class DauServiceImpl implements DauService {
+
+    private final Clock clock;
+    private final ConcurrentHashMap<LocalDate, ConcurrentHashMap<Integer, Set<Integer>>> data = new ConcurrentHashMap<>();
+
+    public DauServiceImpl() {
+        this(Clock.systemDefaultZone());
+    }
+
+    public DauServiceImpl(Clock clock) {
+        this.clock = clock;
+    }
+
+    @Override
+    public void postEvent(Event event) {
+        LocalDate today = LocalDate.now(clock);
+        data.computeIfAbsent(today, k -> new ConcurrentHashMap<>())
+            .computeIfAbsent(event.authorId(), k -> ConcurrentHashMap.newKeySet())
+            .add(event.userId());
+    }
+
+    @Override
+    public Map<Integer, Long> getDauStatistics(List<Integer> authorIds) {
+        LocalDate yesterday = LocalDate.now(clock).minusDays(1);
+        Map<Integer, Long> result = new HashMap<>();
+        var yesterdayData = data.getOrDefault(yesterday, new ConcurrentHashMap<>());
+        for (int authorId : authorIds) {
+            Set<Integer> users = yesterdayData.get(authorId);
+            result.put(authorId, users == null ? 0L : (long) users.size());
+        }
+        return result;
+    }
+
+    @Override
+    public Long getAuthorDauStatistics(int authorId) {
+        LocalDate yesterday = LocalDate.now(clock).minusDays(1);
+        var yesterdayData = data.getOrDefault(yesterday, new ConcurrentHashMap<>());
+        Set<Integer> users = yesterdayData.get(authorId);
+        return users == null ? 0L : (long) users.size();
+    }
+}