initial code

background.js

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+chrome.runtime.onMessage.addListener(function(message, sender) {
+  if (message.content == 'ready') {
+    chrome.pageAction.show(sender.tab.id);
+  }
+});
+
+chrome.pageAction.onClicked.addListener(function(tab) {
+  chrome.tabs.sendMessage(tab.id, {content: "setTimes"});
+});

content.js

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+chrome.runtime.sendMessage(
+  {content: 'ready'}
+);
+
+chrome.runtime.onMessage.addListener(function(message, sender) {
+  if (message.content == "setTimes") {
+    setTimes();
+  }
+});
+
+var setTimes = function() {
+  for (var i=0; i<31; i++) {
+    var day;
+    i + 1 < 10 ? day = "0" + (i + 1) : day = i + 1;
+    var idString = "ctl00_ContentMain_AttendanceTableRepeater_ctl" + day;
+    var startInputId = idString + "_txtStartTime";
+    var endInputId = idString + "_txtEndTime";
+    setValueIfBlank(document.getElementById(startInputId), '09:00');
+    setValueIfBlank(document.getElementById(endInputId), '18:00');
+  };
+};
+
+var setValueIfBlank = function(input, value) {
+  if (input && input.value == "" && input.style['background-color'] == "lightyellow") {
+      input.value = value;
+  };
+};

manifest.json

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+{
+  "manifest_version": 2,
+  "name": "AttendancerXP",
+  "version": "0.1",
+  "icons": {
+    "128": "icon.png"
+  },
+  "background": {
+    "persistent": false,
+    "scripts": ["background.js"]
+  },
+  "content_scripts": [{
+      "matches": ["https://attendance.cvi.co.jp/AttendanceTableFullSimp.aspx"],
+      "js": ["content.js"]
+  }],
+  "page_action": {
+    "default_icon": "icon.png"
+  }
+}