Let us assume that I have a system where temperature, humidity, rain, etc. measurements are recorded at certain dates & times for certain locations (let us call them "snapshots"). 

This is just an example, and such "snapshots" can be any measurements taken on an irregular basis, for example dates when inventory was taken in warehouses, or patient interactions with doctors (some rare, some frequent - and we want to compare some parameters between two interactions), and so forth.

Some locations get snapshots taken on an almost daily basis, but some only once every few months. Some locations used to get them once every few months, but now get them more often. Some may have 3 snapshots, and some may have 50 (but never more as they get pruned).

For a given location, **I want to allow the user to select exactly two different snapshots**, then I will redirect him to a page where I compare them side by side, calculate some other values based on them, and so forth.

**Option A:** I'm considering would be **two calendar controls**, and enabling for selection only the dates on which snapshots were taken (also marking them another colour).

However, the snapshots can be distributed over time. For example, there is a chance that some locations would only have snapshots taken every 6 months. This would force the user to scroll month to month in order to make their selections. 

Also, in rare cases there can be two snapshots made in one day (say, one in the morning and another in the evening), so the user won't be able to select which one exactly he wants.

**Option B:** I'm considering is just have **two drop-downs or multi-line selects listing the available snapshots**; user clicks one, then the other and he's done. This works well for locations with not many snapshots, but will have poor usability for those with many.

Am I missing some simple elegant solution to this?

P.S. I want something fairly simple to implement from ready-made HTML elements / jQuery widgets like datepicker, I don't want to custom-code anything fancy.