Internal format of TDateTime in Delphi 7?

Question

I've spent many hours researching this and am pretty stuck: my question is - has the internal format of a Delphi TDateTime changed between Delphi 7 (released in 2002 or so) and today?

Scenario: I'm reading a binary logfile created by a Delphi 7 app, and the vendor tells me it's a TDateTime in the record, but decoding the bits shows it's clearly not standard IEEE 754 floating point even though the TDateTime produced by modern Delphi is.

But it's some kind of floating point with around 15 bits of exponent and 45 bits of significand (as opposed to 11 and 53 bits in IEE754), and the leading bit is a 1 (which in IEE754 indicates a negative number) for numbers that are clearly not negative, such as the current date/time.

Hints in old documentation suggested that TDateTime "read as" a double but wasn't necessarily represented internally as one, which means that the internal format would be mostly invisible except where these TDateTimes were written out in binary form.

My suspicion is that the change occurred with Delphi 8, which added .NET support, but I simply can't find any references to this anywhere. I have perl code (!) that picks apart these types mostly working, but I'd love to find a formal spec so I can do it properly.

Any old-timers run into this?

~~~ Steve

Answer 1

Nothing has changed since Delphi 7. In Delphi 7, and in fact previous versions, TDateTime is IEEE754, measuring the number of days since the Delphi epoch.

You are going to need to get in touch with the software vendor and try to work out what this data's format really is. It would be surprising if the format was a non-IEEE754 floating point data type. Are you quite sure that it is floating point?

Answer 2

As for BCB3, BCB6 and D4, it's exactly the IEEE 754 Double-precision floating-point format, in the VCL source file system.pas (as included in BCB6) it's defined by thus:

TDateTime = type Double;