diff --git a/README.md b/README.md
index 570f11a..38e6aef 100644
--- a/README.md
+++ b/README.md
@@ -68,6 +68,8 @@ Biz.configure do |config|
 end
 ```
 
+Configured timestamps must be in either `HH:MM` or `HH:MM:SS` format.
+
 Shifts act as exceptions to the hours configured for a particular date; that is,
 if a date is configured with both hours-based intervals and shifts, the shifts
 are in force and the intervals are disregarded.
diff --git a/lib/biz/day_time.rb b/lib/biz/day_time.rb
index 9916f95..4dd44e7 100644
--- a/lib/biz/day_time.rb
+++ b/lib/biz/day_time.rb
@@ -38,7 +38,10 @@ def from_timestamp(timestamp)
               match[:minute].to_i * Time.minute_seconds +
               match[:second].to_i
           )
-        }
+        } or fail(
+          Error::Configuration,
+          'invalid timestamp: must be in `HH:MM` or `HH:MM:SS` format'
+        )
       end
 
       def midnight
diff --git a/spec/day_time_spec.rb b/spec/day_time_spec.rb
index fa974d2..4378e21 100644
--- a/spec/day_time_spec.rb
+++ b/spec/day_time_spec.rb
@@ -78,33 +78,63 @@
   end
 
   describe '.from_timestamp' do
-    context 'when the timestamp is malformed' do
+    context 'when the timestamp is not a timestamp' do
       let(:timestamp) { 'timestamp' }
 
-      it 'returns nil' do
-        expect(described_class.from_timestamp(timestamp)).to eq nil
+      it 'raises a configuration error' do
+        expect {
+          described_class.from_timestamp(timestamp)
+        }.to raise_error Biz::Error::Configuration
+      end
+    end
+
+    context 'when the timestamp is in `H:MM` format' do
+      let(:timestamp) { '5:35' }
+
+      it 'raises a configuration error' do
+        expect {
+          described_class.from_timestamp(timestamp)
+        }.to raise_error Biz::Error::Configuration
       end
     end
 
-    context 'when the timestamp is well formed' do
-      context 'without seconds' do
-        let(:timestamp) { '21:43' }
+    context 'when the timestamp is in `HH:M` format' do
+      let(:timestamp) { '12:3' }
 
-        it 'returns the appropriate day time' do
-          expect(described_class.from_timestamp(timestamp)).to eq(
-            described_class.new(day_second(hour: 21, min: 43))
-          )
-        end
+      it 'raises a configuration error' do
+        expect {
+          described_class.from_timestamp(timestamp)
+        }.to raise_error Biz::Error::Configuration
       end
+    end
 
-      context 'with seconds' do
-        let(:timestamp) { '10:55:23' }
+    context 'when the timestamp is in `HH:MM:S` format' do
+      let(:timestamp) { '11:35:3' }
 
-        it 'returns the appropriate day time' do
-          expect(described_class.from_timestamp(timestamp)).to eq(
-            described_class.new(day_second(hour: 10, min: 55, sec: 23))
-          )
-        end
+      it 'raises a configuration error' do
+        expect {
+          described_class.from_timestamp(timestamp)
+        }.to raise_error Biz::Error::Configuration
+      end
+    end
+
+    context 'when the timestamp is in `HH:MM` format' do
+      let(:timestamp) { '21:43' }
+
+      it 'returns the appropriate day time' do
+        expect(described_class.from_timestamp(timestamp)).to eq(
+          described_class.new(day_second(hour: 21, min: 43))
+        )
+      end
+    end
+
+    context 'when the timestamp is in `HH:MM:SS` format' do
+      let(:timestamp) { '10:55:23' }
+
+      it 'returns the appropriate day time' do
+        expect(described_class.from_timestamp(timestamp)).to eq(
+          described_class.new(day_second(hour: 10, min: 55, sec: 23))
+        )
       end
     end
   end