Most likely your custom content type is deriving from
Item content type. You need to inherit it from
Event content type. If you are adding a content type using Visual Studio, you are presented with an option to select the parent content type as shown:
However, if you already have created it, you can still modify the Elements.xml file. Id of
0x0102. Append this value in ID property of Content Type followed by the GUID. Example:
?xml version="1.0" encoding="utf-8"?>
<!-- Parent ContentType: Event (0x0102) -->
Group="Custom Content Types"
Description="My Content Type"