I managed to verify the content via the function: "CELL":
=CELL("format", A1)
Gives an output dependent the cell content.
I expect something like: DD.MM.YYYY hh:mm:ss
and this correspond to the function output: "D1"
A cell format of "hh:mm:ss" gives: "U4"
It might be that the function is sensitive to your localization,
I have tested it with a German localization, the German variant is:
'ZELLE("format";B18)