Using the ST-LINK Utility to configure the RTC to current time
Some registers are only legal for a specific access width (ie, -w32 may not be correct), or may not read back the written values which could cause a problem with verification.

There might also be sequence or state restrictions on accessing things.

An option that should work around most conceivable issues would be to craft a tiny program to do the job which would linked to run in RAM.  You could substitute the data into its binary after figuring out the offset, upload the modified version, and run it.  Or you could have the program get values from a region of RAM outside of the file's extents, which you would set before running.  With finer grained control of the stlink you could also pass values in CPU registers, though you might(?) need the alternate open source command line program rather than ST's to do that (this small routine in RAM method is incidentally how that program accomplishes writing to flash)