This is indeed hardcoded in old BG2. Journal formatting is done by the function at 0x0056FC9A. What it does is the following:
* fetch tlk entry for requested journal entry
* find first newline character
* everything before that becomes "caption" in the next gui entry (slightly different color, etc.)
* build timestamp string based on 15980 format pattern (replacing <DAY> etc with actual values for the entry)
* append everything starting from first newline character to the timestamp string - it becomes "text" in the next gui entry
So depending on what exactly you want to achieve, you might need to modify the code.
If all you need is to add space between caption and date string, you can modify 15980 tlk entry by adding newline character at the very beginning (but note that it might cause some unexpected changes in other places 15980 is used).