Matt,
Housekeeping first; Clarion is a language, development system, data file format (DAT), and the name of the originating company (actually it had 1 earlier name but it doesn't matter). TopSpeed is the name of a company that was a merger result of Jensen and Partners International (TopSpeed compilers) from the UK and Clarion Software from the US. The TopSpeed file format (TPS) is also an ISAM data file.
Collectively all things Clarion and TopSpeed (software not developers/companies) are owned by a completely new company; SoftVelocity.
Date/Time fields are magic in Clarion! Actually they are stored in standard Clarion Date and Clarion Time formats.
Standard Date: Elapsed days since December 28, 1800. The range of accessible dates is from January 1, 1801 (standard date 4) to December 31, 9999 (standard date 2,994,626)
Standard Time: Elasped 100th/sec since midnight The range is 1 (defined as midnight) to 8,640,000 (defined as 11:59:59.99 PM)
Now to the disappointing bit, the Clarion DAT file format has actually been published (no I can't locate it). The format has changed at least once and they are incompatible. Your basic premise is correct that there is a header block and it describes the layout, width and quantity of records. If the file has been Owner encrypted you won't be able to tell much in clear text. The task you are embarking on is not impossible just very difficult.
Is this a one time deal or are you looking to access the data on a continuing basis?
HTH