How do you edit fields (and in particular the DATE field) in FITS header ?
Invoking the “FITS Header” action in the Tools menu displays the header but the values don’t seem to be editable (clicking has no effect).
Moreover, when editing the header with an external editor and reloading the image, the new DATE field does not seem to be taken into account (its value seems to be stuck the file creation date of the FITS file).
It might help if you tell us what program you are using (and which version). It could be that the program just uses the file creation date, and not the date in the header. But with what you (don’t) tell us, we can only guess about that.
Also, if a field isn’t editable, there can be a good reason for it (e.g. in normal operation there’s no need to change the field, or changing it causes issues with tracability).
Sorry for the late answer (the notification got in the junk mail subdir).
Sorry also for not having been informative enough.
I’ve tried to use Siril (1.4.2) to perform astrometric resolution (and asteroid identification - using IMCCE-based annotation) on a (old) PNG image which has been produced by another image processing software. Of course, to do this, the acquisition date/time (and approx position) of the image is required.
My idea was therefore either to 1) load the PNG image 2) edit the FITS to add/fix the required information - or 1) load the PNG image 2) save it as FITS 3) edit the FITS header with an external editor 4) reload the FITS.
Thanks for pointing this. I now understand the problem : the field i was trying to edit was “DATE” (documented as “UTC date that the FITS file was created”) and not “DATE-OBS” ("observation start, UTC). This clearly makes sense that the latter is editable and the former not 
I should have paid more attention to the “Comment” field of the header edit panel !
Thanks for your help anyway