Using FME 2018.1

I suspect this is a feature of ArcCatalog and the 00:00:00 is just not displayed!

If you want to be sure FME is writing the midnight time, run the workspace with "Redirect to FME Data Inpsector" (under the writers menu) turned on.

FME will write to the Data Inspector instead of to the database, so you can check the data before the writing.

That will allow you to see what is actually written, before ArcCatalog is displaying it and will allow you to see if the 00:00:00 is written.

I'm not so sure about that. I just tried it, read it back with FME, and the time was missing. I think this might be our issue. I'll check with our developers and get back to you. Are you using the full geodatabase writer or the open API one? It was the API one I tried.

So... it appears to be a known limitation that 00:00:00 in Geodatabase represents "no time". So I don't think it's going to be possible to retain a meaningful midnight value!

I don't think it supports fractions of a second either, so the best you could do is add a second to make it 00:00:01, using the DateTimeCalculator transformer.

I did try fractions of a second and they just disappeared too We can live with this as we want the date times in sequence and this will still sort correctly . Eventually we are going directly to Arcgis Online, I will check what happens there as soon as I can

It's an ESRI thing rather than FME. Try and set the time to 00:00:00 directly within the GDB and you won't see the 00:00:00 displayed. It assumes a midnight time if one isn't shown, e.g. if you query for = timestamp '2019-10-09 00:00:00' it will display those records where 00:00:00 isn't displayed