diff --git a/tests/utils.test.js b/tests/utils.test.js
new file mode 100644
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--- /dev/null
+++ b/tests/utils.test.js
@@ -0,0 +1,76 @@
+/*
+	Neonatal jaundice treatment threshold calculator
+	Copyright (C) 2024  Lee Yingtong Li
+	
+	This program is free software: you can redistribute it and/or modify
+	it under the terms of the GNU Affero General Public License as published by
+	the Free Software Foundation, either version 3 of the License, or
+	(at your option) any later version.
+	
+	This program is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU Affero General Public License for more details.
+	
+	You should have received a copy of the GNU Affero General Public License
+	along with this program.  If not, see <https://www.gnu.org/licenses/>.
+*/
+
+// Import bilirubin_lib.js
+const fs = require('fs');
+eval(fs.readFileSync('src/bilirubin_lib.js').toString());
+
+const assert = require('node:assert');
+const { suite, test } = require('node:test');
+
+suite('Hours display', () => {
+	test('0 hours', () => {
+		assert.equal(prettyPrintHours(0), '0 hours');
+		assert.equal(prettyPrintHours(0.2/24), '0 hours');
+		assert.equal(prettyPrintHours(0.49/24), '0 hours');
+	});
+	
+	test('1 hour', () => {
+		assert.equal(prettyPrintHours(0.5/24), '1 hour');
+		assert.equal(prettyPrintHours(1/24), '1 hour');
+		assert.equal(prettyPrintHours(1.49/24), '1 hour');
+	});
+	
+	test('2-23 hours', () => {
+		for (let h = 2; h <= 23; h++) {
+			assert.equal(prettyPrintHours((h - 0.5) / 24), h + ' hours');
+			assert.equal(prettyPrintHours(h / 24), h + ' hours');
+			assert.equal(prettyPrintHours((h + 0.49) / 24), h + ' hours');
+		}
+	});
+	
+	test('>= 24 hours', () => {
+		assert.throws(() => prettyPrintHours(23.5/24));
+		assert.throws(() => prettyPrintHours(23.99/24));
+		assert.throws(() => prettyPrintHours(1));
+		assert.throws(() => prettyPrintHours(1.1));
+		assert.throws(() => prettyPrintHours(7));
+	});
+});
+
+suite('Days and hours display', () => {
+	test('0 days', () => {
+		assert.equal(prettyPrintDays(0), '0 hours');
+		assert.equal(prettyPrintDays(0.5/24), '1 hour');
+		assert.equal(prettyPrintDays(23.49/24), '23 hours');
+	});
+	
+	test('1 day', () => {
+		assert.equal(prettyPrintDays(1), '1 day, 0 hours');
+		assert.equal(prettyPrintDays(1 + 0.51/24), '1 day, 1 hour');
+		assert.equal(prettyPrintDays(1 + 23.49/24), '1 day, 23 hours');
+	});
+	
+	test('2-14 days', () => {
+		for (let d = 2; d <= 14; d++) {
+			assert.equal(prettyPrintDays(d), d + ' days, 0 hours');
+			assert.equal(prettyPrintDays(d + 0.51/24), d + ' days, 1 hour');
+			assert.equal(prettyPrintDays(d + 23.49/24), d + ' days, 23 hours');
+		}
+	});
+});