To make an SQL query like:
SELECT id FROM Account WHERE LastActivityDate = 30_DAYS_AGO
You can calculate the date in apex and then bind it into your query.
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As shown in Example:
date d = system.today().addDays(-30); Account [] acc= [select id from account where createdDate = :d];
Or, you can go ahead and use:
SELECT id FROM Account WHERE LastActivityDate = LAST_N_DAYS:30
LAST_N_DAYS is a sytax which is used to find out the activity in the last N days (here, N = 30)
