Do not have any experience with this software, but since it runs on Windows, it might be a large int format. Almost all OSes use some int format for datetime with ticks since an epoch. The tick and epoch for Windows is given below.

Windows datetime: DateTime Values and their string representations

Internally, all DateTime values are represented as the number of ticks (the number of 100-nanosecond intervals) that have elapsed since 12:00:00 midnight, January 1, 0001. The actual DateTime value is independent of the way in which that value appears when displayed in a user interface element or when written to a file. The appearance of a DateTime value is the result of a formatting operation. Formatting is the process of converting a value to its string representation.

https://msdn.microsoft.com/en-us/library/system.datetime(v=vs.110).aspx.

Take a look on proteowizard, it converts many mass spec formats to a easy to access format

http://proteowizard.sourceforge.net/

re Gerard: It doesn't seem to be a large int.

re Ricardo: ProteoWizard does MassHunter .d but not Chemstation .D

Thanks for your time anyway

updated gist

https://gist.github.com/lorenzgerber/f840249930d186b71a0b