As nobody seems to know of any tool around, I decided to create one myself.
I've used the "GBA rtcread" tool as base code for GUI, so if you used that tool before, you should be familiar with it.
Just be advised that you are responsible to sync the "week day" value too when changing the Date.
I was able to successfully do in-game events without penalties, like growing berries by changing time in increments of 4 hours, and saving again.
This tool changes the RTC without modifying the "RTC Offset" data in the firmware, so games cannot detect time changes.
Source code is included.
RTCChangerNDS.zip