Let's you have a requirement to track the page modification information in a SharePoint site, there are many ways to do it. I would suggest the simpler and easier way.
Steps
1. Edit the Master page in SharePoint designer.
2. Add the below code
<%@ Register TagPrefix='SharePointWebControls'
Namespace='Microsoft.SharePoint.WebControls'
Assembly='Microsoft.SharePoint, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c' %>
<SharePointWebControls:EncodedLiteral runat='server'
Text='<%$Resources:cms,pagesettings_modifieddate_label%>'
EncodeMethod='HtmlEncode'/>:
<SharePointWebControls:DateTimeField runat="server"
FieldName="Modified"
ControlMode="Display"/>
3. Here mode should be Display since the control should be visible only on display mode.
Hope it helps
Happy Share Pointing :-)