date+1

[HeatherNewton]

what does

put(fdate+1,date9.) mean?

what does it mean to add 1 here?

say fdate is 31Aug2021

[PeterClemmensen]

A SAS Date is an integer. So adding 1 will give you the next day.

The put statement converts the numeric SAS variable into character.

[mkeintz]

Assuming fdate is a date value (i.e. number of days since 01jan1960), then to represent 31aug2021, the underlying value of date is 22523.  That can be revealed by running this code:

data _null_;
   fdate='31aug2021'd;
   put fdate=;
   put fdate=date9. ;
run;

which generates this log:

6761  data _null_;
6762    fdate='31aug2021'd;
6763    put fdate=;
6764    put fdate=date9.;
6765  run;

fdate=22523
fdate=31AUG2021
NOTE: DATA statement used (Total process time):
      real time           0.00 seconds
      cpu time            0.00 seconds

But you ask about the put function (unlike the put statements above).  AS @PeterClemmensen says, it calculates the first argument (i.e. adding 1 to the value 22523, getting 22524), and converts it to a character string using the date9. format, yielding the character "01sep2021":

data _null_;
  fdate='31aug2021'd;
  z=put(fdate+1,date9.);
  put z=;
run;

Run these programs and take a look at the log.

[AMSAS]

Here's the documentation links with good examples

PUT Function 

Working with Dates in the SAS System