Remote Desktop Service license - 2008R2

We have 50 users RDS licenses installed. The license report indicated each user has different expiration date/time. What does that mean "Exires on"?

CAL Usage Report
RD License Server:,"LICENSE01"
Report Date:,"‎Friday, ‎August ‎09, ‎2013 12:59:17 AM"
Report Scope:,"Domain"

Issued to User,CAL Version,Expires On
domain\user1,Windows Server 2008 or Windows Server 2008 R2,‎Monday, ‎October ‎07, ‎2013 5:37:55 AM
domain\user2,Windows Server 2008 or Windows Server 2008 R2,‎Wednesday, ‎August ‎28, ‎2013 9:50:29 AM
domain\user3,Windows Server 2008 or Windows Server 2008 R2,‎Tuesday, ‎October ‎08, ‎2013 2:20:56 AM

CAL Version,CAL Type,Installed CALs,CALs in Use,CAL Availability
Windows Server 2008 or Windows Server 2008 R2,Per User,60,30,Available

Thanks,

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JohnnyW2012