diff --git a/test/src/helpers/index_timestamp.js b/test/src/helpers/index_timestamp.js
index 01c92c4..cb4bce1 100644
--- a/test/src/helpers/index_timestamp.js
+++ b/test/src/helpers/index_timestamp.js
@@ -5,15 +5,15 @@ import indexTimestamp from '../../../lib/helpers/index_timestamp';
 
 const anchor = '2016-04-02T01:02:03.456'; // saturday
 
-describe('Index interval', function () {
-  describe('indexTimestamp construction', function () {
+describe('Index timestamp interval', function () {
+  describe('construction', function () {
     it('should throw given an invalid interval', function () {
       const init = () => indexTimestamp('bananas');
       expect(init).to.throwException(/invalid.+interval/i);
     });
   });
 
-  describe('indexTimestamp timestamps', function () {
+  describe('timestamps', function () {
     let clock;
 
     beforeEach(function () {
@@ -24,34 +24,53 @@ describe('Index interval', function () {
       clock.restore();
     });
 
-    it('should return the year', function () {
-      const timestamp = indexTimestamp('year');
-      expect(timestamp).to.equal('2016');
+    describe('formats', function () {
+      it('should return the year', function () {
+        const timestamp = indexTimestamp('year');
+        expect(timestamp).to.equal('2016');
+      });
+
+      it('should return the year and month', function () {
+        const timestamp = indexTimestamp('month');
+        expect(timestamp).to.equal('2016-04');
+      });
+
+      it('should return the year, month, and first day of the week', function () {
+        const timestamp = indexTimestamp('week');
+        expect(timestamp).to.equal('2016-03-27');
+      });
+
+      it('should return the year, month, and day of the week', function () {
+        const timestamp = indexTimestamp('day');
+        expect(timestamp).to.equal('2016-04-02');
+      });
+
+      it('should return the year, month, day and hour', function () {
+        const timestamp = indexTimestamp('hour');
+        expect(timestamp).to.equal('2016-04-02-01');
+      });
+
+      it('should return the year, month, day, hour and minute', function () {
+        const timestamp = indexTimestamp('minute');
+        expect(timestamp).to.equal('2016-04-02-01-02');
+      });
     });
 
-    it('should return the year and month', function () {
-      const timestamp = indexTimestamp('month');
-      expect(timestamp).to.equal('2016-04');
-    });
+    describe('date separator', function () {
+      it('should be customizable', function () {
+        const separators = ['-', '.', '_'];
+        separators.forEach(separator => {
+          const str = `2016${separator}04${separator}02${separator}01${separator}02`;
+          const timestamp = indexTimestamp('minute', separator);
+          expect(timestamp).to.equal(str);
+        });
+      });
 
-    it('should return the year, month, and first day of the week', function () {
-      const timestamp = indexTimestamp('week');
-      expect(timestamp).to.equal('2016-03-27');
-    });
-
-    it('should return the year, month, and day of the week', function () {
-      const timestamp = indexTimestamp('day');
-      expect(timestamp).to.equal('2016-04-02');
-    });
-
-    it('should return the year, month, day and hour', function () {
-      const timestamp = indexTimestamp('hour');
-      expect(timestamp).to.equal('2016-04-02-01');
-    });
-
-    it('should return the year, month, day, hour and minute', function () {
-      const timestamp = indexTimestamp('minute');
-      expect(timestamp).to.equal('2016-04-02-01-02');
+      it('should throw if a letter is used', function () {
+        const separator = 'a';
+        const fn = () => indexTimestamp('minute', separator);
+        expect(fn).to.throwException();
+      });
     });
   });
 });