Restricting Modified Date value with System Account

Hi,

We have a document library called testdoc.

Now requirement is not to update 'Modified Date' whenever System Account updates the column.

Basically Modified Date is default sharepoint column where it is updated if document is modified by any user?

Now please let me know how to restrict the same if System Account modied any file in the document library?

Regards,

Sudheer

Thanks & Regards, Sudheer