Thanks for the replies.
-->> I'm not sure I understand what 'Time and Date' means? In what application are you seeing the 'Time and Date'?
About halfway down the IPTC Stationary Pad / Caption dialog for the saved image there is a field "Time and Date". You have provided a drop down menu for this to include 'Date', 'Capture Time' and so forth but changing these does not give the option of setting the original date and time.
-->> When you save as, do you have it keep the embedded exif data (that's where the capture datetime is)?
No I don't so I guess this could be the cause and a quick test has certainly shown it does resolve the issue.
However I do set the Stationary Pad details for the Capture Time when I am doing the original work on the file such as editing and selecting and have always thought this was part of what was then embedded. I for one would much prefer that my chosen date and time option were retained as I have the option to change them for whatever reason via PhotoMechanic. Whilst your explanation makes sense, and as I have written seems to offer a solution, it does appear odd that not embedding the EXIF data at a later point can change the details in a field in the caption dialog.
To make it clear these files, from 1D MkII, 1DMIIN and 5DMkII (eg so different models), have satisfactorily been ingested, captioned, renamed, edited in Photoshop and then the caption and filename details been completed. Up to this point the details are correct. It is when I attempt to resave them that the problem occurs.