The saying of honest as the day is long means that a person is very honest and very trustworthy.
If someone says you're honest as the day is long it means you're a very trustworthy and honest person.
The phrase honest as the day is long has been around since the 1900's although the saying of honest is the day is long could have originated years before that as well.
The honest as the day is long phrase also seems to be of fairly recent origin as it was used in print by Richard Shattuck in 'The Snark was a Boofum.
If someone says someone is honest as the day is long it means that the person is a good person and can be easily trusted.
So if someone told you that you're honest as the day is long it's a good thing.
It doesn't make much sense but a lot of phrases and idioms don't make sense.