Answer by John Willemse didn't work for me - it turns out fileTimeUnchanged had to be set to 0xFFFFFFFFFFFFFFFF rather than 0xFFFFFFFF. This is because (according to the docs) you need to pass the FILETIME structure with both DWORDs set to 0xFFFFFFFF. Passing 0xFFFFFFFF as the entire structure would give dwLowDateTime the correct value, but dwHighDateTime would be 0.