Best way to store timestamp HH:MM:SS into eVar

I don't know if there is a "best practice" per se... this is a pretty niche requirement... 

Adobe Workspaces provide breakdowns to a minute granularity already... Data Warehouse exports only support down to hourly data.

For most companies, if they need down to the second reporting, then they generally set up Raw Data feeds (which contain the full and complete timestamp (day, hour, minute, second, millisecond) and process the data (as close as possible to Adobe) and use their own internal data lake).

That said, if you don't want to go that route, and instead set up custom tracking, how you store the data will really depend on how you plan to use it.

Key items to keep in mind:

• How do you want it to display - visually appealing / readable to your users is important 
• Are you going to use Classifications to process the data, and make segments from those classifications? If so, you want values that you can easily parse with Regex
• Any data collected manually like this won't line up with Adobe's timestamp... especially the seconds.... there will always be minor variations (since system time variance is "acceptable" is a 5 min tolerance), and then you have people that don't use auto-sync time that could have anything set on their device.....  Adobe tracking uses the Adobe server to set the timestamp, so it's one consistent source of truth