The reason why you see strange dates in the plot x-axis is that the observation numbers you are using for ID variables are intepreted as SAS dates and reformatted according to the interval specified. SAS dates are number of days since a cutoff data, which by default is 1960. If you run the following examples you will see what I mean. The "dateid" variable is formatted as date9., which is the default for DAY interval, so it prints as 02JAN1960 and so on.
data air;
set sashelp.air;
id = _n_;
dateid = id;
newdateid = id-"01Jan1960"d - 1;
format dateid date9. newdateid day9.;
run; proc print data=air(obs=32); run;
SAS dates and formats are explained in more details here:
http://support.sas.com/documentation/cdl/en/etsug/68148/HTML/default/viewer.htm#etsug_tsdata_sect006.htm
I don't have access to FS as I am writing but I don't think you can remove the format that is assigned automatically according to the INTERVAL specified. Someone else may correct me if I am wrong.
As you see in the example above, I created a new variable "newdateid" that I recentered around 01Jan1960 and formatted as day9. If you use newdateid as your ID variable it will show the day of the month only on the x-axis, which is closer to what you want. However that's not a perfect solution since it will start again from 1 when February starts, and so on for every month.
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