It varies based on RDBMS, but if I guess right, that's PostgreSQL, in which case the :: converts a.dc to a date type of date.

In other flavors...

In MS SQL Server 2000:

For built-in user-defined functions that return a table, the function name must be specified with a leading double colon (::) to distinguish it from user-defined functions that are not built-in. It also must be specified as a one-part name with no database or owner qualifications. For example: SELECT * FROM ::fn_helpcollations() b.. For built-in user-defined functions that return a scalar value, the function name must be specified as a one-part name (do not specify database or owner). Do not specify a leading double colon (::).

In MS SQL Server 2005:

Double-colons are no longer required for UDFs that return a table.

However...

Double-colons are required in SQL Server 2005 when granting permissions on schemas, certificates, endpoints, and a few other securables.

As well as...

When using User-Defined Types, static methods of the type must be called using the double-colon syntax.

Sources: BOL and Kalen Delaney's Blog

In this case, it is a cast to a date type. :: is a type cast that can also be represented as CAST(expression AS type).

It is a CAST operation(cast to a date type).

Example:

SELECT now()::timestamp(0);

Is equivalent to:

SELECT 
    CAST (now() AS timestamp(0));

They both result in casting now() to timestamp in the following format: YYYY-MM-DD HH:MM:SS

It is probably a cast, converting a.dc to type date.

IBM Informix Dynamic Server (IDS) would work that way - but the INTERVAL notation at the end is not valid for IDS, so presumably this is in fact another DBMS (probably PostgreSQL).