I don't think you need a macro here. Some nested built-in function will do.
Formula for starting interval:
=TEXT(DATE(YEAR(TODAY()),MONTH(TODAY())+1,1),"yyyymm")
This basically returns a date formatted as "yyyymm" of the next month from today
Formula for ending interval:
=IF(MONTH(TODAY())>4,TEXT(DATE(YEAR(TODAY())+1,12,1),"yyyymm"),TEXT(DATE(YEAR(TODAY()),12,1),"yyyymm"))
This checks if today's month is greater than 4, if so move end interval to next year's december otherwise keep it as this year's december.
There are several formulas used here:
TEXT(value,format) - this returns a formatted text
DATE(YEAR,MONTH,DAY) - this returns a date
YEAR(date_serial) - this returns the year from a date serial
MONTH(date_serial) - this returns the month from a date serial
DAY(date_serial) - this returns the day from a date serial
IF(logic,true,false) - if statement in Excel
TODAY() - this returns today's date serial