Hi artemike,
The only way to set the date and time is to connect the SX700 to the local WiFi router using the Yamaha WiFi stick "UD-WL01" (available separately). Note that these settings can be lost if the keyboard is not switched on for several days.
Basically, I don't think it's particularly important that files created on the keyboard (e.g. recordings) have the current date / time. Just rename the files, e.g. with the song name, and you will find everything.
Best regards,
Chris