String query = 'Select Id From Account Where Membership_Expiration_Date__c = ' + System.today().addDays(30);

...String.valueOf(System.today().addDays(30)).substringBefore(' ');

public static String getSoqlDate(Integer daysFromNow)
{
    return Datetime.now().addDays(daysFromNow).formatGmt('yyyy-MM-dd');
}
public static String getSoqlDatetime(Integer daysFromNow)
{
    return Datetime.now().addDays(daysFromNow).formatGmt('yyyy-MM-dd'T'HH:mm:ss.SSS'Z'');
}