The RTC in the Signature, like in the legacy instruments, cannot be set to fractions of a second. There is a simple solution to your problem though, you can enable time synchronization over Ethernet whereby we synchronize the clock to an external clock reference, either PTP or NTP. If you are connected to Internet you can specify the IP address of a time server directly (see the help text in the right hand column of the Signature web interface on how to do this). If the Signature is not connected to Internet you can instead configure your PC as an NTP server and synchronize the Signature to the PC. Here is a link that describes how to configure this on your PC:

https://www.interfacett.com/blogs/creating-standalone-ntp-server-windows/