Refactors the header

components/header/title.js

@@ -0,0 +1,30 @@
+import React from 'react'
+import { View, Text} from 'react-native'
+import PropTypes from 'prop-types'
+
+import styles from '../../styles'
+
+export default function Title({ title, subtitle }) {
+
+  if (subtitle !== undefined) {
+    return (
+      <View>
+        <Text style={styles.dateHeader} testID='headerTitle'>
+          {title}
+        </Text>
+        { subtitle &&
+          <Text style={styles.cycleDayNumber} testID='headerSubtitle'>
+            {subtitle}
+          </Text>
+        }
+      </View>
+    )
+  }
+
+  return <Text testID='headerTitle' style={styles.headerText}>{title}</Text>
+}
+
+Title.propTypes = {
+  title: PropTypes.string,
+  subtitle: PropTypes.string,
+}