ControlLogix GSV & SSV instruction

All
I am studying a PLC program from another facility. It uses an SSV instruction and a GSV. Data in SSV, GSV as follows.
-----------SSV---------------------------
| Set System Value:
| Class Name: WALLCLOCKTIME
| Instance Name:
| Attribute Name: DateTime
| Source: MES_WALLCLOCKTIME_NEWVALUE[0]
------------------------------------------

----------GSV----------------
| Class Name: WALLCLOCKTIME
| Instance Name:
| Attribute Name: DateTime
| Dest: PLCSystemDateTime[0]
-----------------------------
The only one of these names that is a tagname is the destination and source. What are these for??

The SSV is setting the PLC clock using the information stored in MES_WALLCLOCKTIME_NEWVALUE. This tag is an array, each element will contain a piece of the processor time (month, day, year and so on)

The GSV is retrieving the PLC clock time and storing it in PLCSystemDateTime. Again it's an array, each element will contain a piece of the time.

Thanks!
But Are either of the Dest. or source tags a status tag that is being over written or both created by original programmer?
And what is the significance of the Instance Name and Class Name?

Both tags are created by the original programmer.

The tag that is in the source of the SSV is being "read" by the SSV, then passed to the processor clock. The class, instance and attribute are determining essentially where the data is going.

The GSV is reading processor clock and then writing the value into the destination tag. If you try and modify the values the destination tag, the GSV will overwrite your changes (assuming that the GSV is being scanned all the time).

You can click on the instruction in the program and hit F1 to bring up the help on it.

Again the class, instance and attribute in those instances are determining that you are working with the PLC clock. There are other pre-defined settings for getting fault information, status of modules and so on. The help file will tell you more about them.

if you want to read up on all the details, get this:

http://literature.rockwellautomation.com/idc/groups/literature/documents/pm/1756-pm015_-en-p.pdf