The Last Modified Date is maintained by the ASM2 OPEN Modification. The $US function reads the FMT1 DSCB and expects it to have the format re-written by the OPEN MOD.
The name of the OPEN Modification is L1OMVS6W (Usermod(CL14201)
The information for the usermod is located in the Getting Started Guide section 2.15 Step Open Modification
In the ASM2 System Reference Guide
12.1 Data Set Usage - $US
The $US command displays data set usage information contained in the VTOC (or VSAM catalog) for specified data sets or any ageable VSAM object. $US can be issued interactively in batch mode, as a TSO command, or from the ISPF Disk Space Management Utility panel (see the IPSF User's Guide for details). Specifying a high-level node or a generic subset of a node displays usage information for a group of data sets. Usage information for VSAM objects not known to the current catalog environment is not available. Usage data consist of the following:
■ Total usage
■ Date of last use
■ Date, time, and user ID of the last update
Note: This usage data is only available when the CA-ASM2 OPEN modification is installed.
See page 4-72 in the Getting Started Guide section for additional information regarding this modification.