Documentation is the answer
You can use calculations in file names
Page 92
GETSYSTEMVARIABLE('SYSTEM_DATE') will return current date in ‘YYYYMMDDHHNNSS’ format
If we want only part of date we can use LeftString(String,Count):String for example
LeftString(GETSYSTEMVARIABLE('SYSTEM_DATE'),8) will return only date part.
Now more complicated example for loading yesterday’s data;
LeftString(DecDateS(GETSYSTEMVARIABLE('SYSTEM_DATE'),'YYYYMMDDHHNNSS', 'DAY',1),8)