diff --git a/src/helper.cpp b/src/helper.cpp
index 111afd54..d0a6cb74 100644
--- a/src/helper.cpp
+++ b/src/helper.cpp
@@ -173,3 +173,21 @@ Interval getLatestInterval (
 }
 
 ////////////////////////////////////////////////////////////////////////////////
+// An interval matches a filter if the start/end overlaps, and all filter tags
+// are found in the interval.
+bool intervalMatchesFilter (const Interval& interval, const Filter& filter)
+{
+  if (interval.end () > filter.start () &&
+      interval.start () < filter.end ())
+  {
+    for (auto& tag : filter.tags ())
+      if (! interval.hasTag (tag))
+        return false;
+
+    return true;
+  }
+
+  return false;
+}
+
+////////////////////////////////////////////////////////////////////////////////
diff --git a/src/timew.h b/src/timew.h
index bd94c775..516dbfcf 100644
--- a/src/timew.h
+++ b/src/timew.h
@@ -50,6 +50,7 @@ void expandIntervalHint (const std::string&, std::string&, std::string&);
 Filter createFilterFromCLI (const CLI&);
 Timeline createTimelineFromData (const Rules&, Database&, const Filter&);
 Interval getLatestInterval (Timeline&, Database&, const Filter&);
+bool intervalMatchesFilter (const Interval&, const Filter&);
 
 // utiŀ.cpp
 std::string osName ();