Find recurrent dates in a small set (and get rid of non recurrent ones) I need help in the analyse of a categorization problem.

Given a set of dates (small set: 20 elements maximum), I would like to group dates which are equally distributed (with a tolerance). 
It can be, for instance, monthly or weekly separated dates.

Here is an example. Given this repartition : 

I would like to categorize into these two groups: 

The problem is that I am a developer, not a data scientist. I have an intuition that it should be possible to do a kind of regression.

I have no clue how to analyse this problem. Can you help me with that, please ?

Cheers

PS: I have already seen this thread (Recurring events - finding in a time series) but I have not helped me.
If it is a categorization problem then you should look for a classification algorithm, not a regression technique. The simplest classification algorithm is Logistic Regression. 

But by the looks of it, seems like you do not have a labelled data-set and if that's the case you should look for Clustering techniques. Clustering is a part of Unsupervised Learning Technique in ML which create clusters or groups of similar data points.