Should Android kernel version date match the release date? In the process of checking the settings on my Sony Xperia XA I came across the Linux Kernel information. Version 3.18.35 was released in June 2016 but the date shown is from 2017:

3.18.35+
BuildUser@BuildHost #1
Fri Sep 15 05:31 CST 2017


Is it normal for the date shown here to be later than the release date? I assume it may represent the last automatic update. Also, what does the + sign mean next to the version number?. 

I have no idea what BuildUser@BuildHost is but I am hoping someone can reassure me that it is normal for Sony phones :-)
To create the kernel for your phone, Sony downloaded Linux 3.18.35 (released June 2016), modified it to include your device’s drivers and built¹ it on the 15th of September. The date you are seeing in version information is the build date, not the release date, of your device’s kernel.
The plus means changes to the code of the kernel were made without being committed to the git² version control system. (read this answer for more info)
Lastly, BuildUser@BuildHost means that the kernel was compiled by a user called BuildUser on a machine called BuildHost.

¹ Or compiled, which means converting from human-readable C code to processor-runnable machine code
² A version control system. After editing code, you commit it to your local git repository to make a record of the changes you made. Then you sync it to the central git repository so your team members receive the changes you made.