This is my code:
return Newsletterctx.Subscribers.Count(o => o.Validated == false && o.ValidationEmailSent == true && o.Su
Perhaps you can shift the date to test against instead:
DateTime testDate = DateTime.Now.AddMonths(-1); return Newsletterctx.Subscribers.Count (o => o.Validated == false && o.ValidationEmailSent == true && o.SubscriptionDateTime < testDate);