I want to know how many tweets were posted with the hashtag
from March 1st 2015 - March 31st 2015
Since we do not have access to 100% of tweets, we can't answer that. Our value prop for just the engagement analytics is showing you the historical Exposure/hour and Retweets/hour (which has the last hour and the last day's stats factored in) versus Unique Tweets/hour (your competition.)
For 100% of Tweets, say, when you want to reply to all people who used a campaign hashtag, you might want Hashtags.org - but then, they do this for the future Tweets, not past - and I think they charge about US$50 per hashtag.
Hope this helps,
Customer support service by UserEcho