We don't have a date function in ADQL, instead we use the UNIX timestamps. However with Custom Time Range option available on the dashboards too you should be able to see 1 month time range data without any issues(provided that you have data retention of last 1 month or greater so that the data exists at the backend and isn't rolled over). Could you please let us know what issue you are facing trying to use custom time ranges so that we may try to see if there is a possible solution.
In any case if that doesn't work for you, you may try using the ADQL query with UNIX eventtimestamp range and get 1 month data. So assuming you have data retention for the required time period, you can run an ADQL query like below:
SELECT series(eventTimestamp, '30d', 1530403200, 1533054268), distinctcount(sessionguid) FROM web_session_records WHERE appkey = "AD-AAB-AAF-XCP" LIMIT 500
This should show you results for 1 month range as passed in your query.
Please note that these time stamps are in UNIX (Epoch timestamp), so you'll need to use appropriate conversion for whatever range you want.
Note2: You can limit your searches for better pagination if larger sets of data if needed to be queried. In this example I am using a LIMIT of 500, so it will list only first 500 records out of all that is fetched. Please refer to the below doc link for better understanding how LIMIT caluse can be used: