Problem Statement

Invoking NOTES without commandline arguments displays the current contents of the local NOTES.TXT if it exists. If NOTES has arguments, the current date and time are appended to the local NOTES.TXT followed by a newline. Then all the arguments, joined with spaces, prepended with a tab, and appended with a trailing newline, are written to NOTES.TXT. If NOTES.TXT doesn't already exist in the current directory then a new NOTES.TXT file should be created.

Let's start with the solution:

IF LEN(COMMAND$) THEN
    OPEN "notes.txt" FOR APPEND AS 1
    PRINT #1, DATE$, TIME$
    PRINT #1, CHR$(9); COMMAND$
    CLOSE
ELSE
    d$ = DIR$("notes.txt")
    IF LEN(d$) THEN
        OPEN d$ FOR INPUT AS 1
        WHILE NOT EOF(1)
            LINE INPUT #1, i$
            PRINT i$
        WEND
        CLOSE
    END IF
END IF