Rather than an "adjust time" control[*], I'd much rather it actually tracked the time zone of the computer and the dive site, doing conversions as necessary.
We do in fact do this for dive computers that support it. For example, most UWatec computers keep UTC time and then the users just set the UTC offset of the place they are diving. And we use that to figure out the time of the dive. I would say that most brands though just deliver a number for the day/month/year etc of the dive. So there is no way to know what time zone the dive took place in.
For example, I'm primarily a travel diver so I just keep my computer set to UTC,
I think your's is a unique use case. I think most manufacturers expect and most divers do set their dive computers time to the local time where they are diving. *Especially* those that travel to dive. Since almost all of them show the "time of day", most people want that to be the "local" time of day, not the time of day somewhere else - home or otherwise. Certainly we've all forgotten to change the time on our dive computers before the first dive of a trip at one time or another, but this is considered a "mistake" and not practice. I'd guess that you keep yours in UTC time so that you don't
have to remember to set it before each 1st dive of a trip. I have just recently seen some new models where you can have 2 separate times set so you can have both local time and "home" time displayed on it, but again, this is fairly new.
I've found that most other logbook software will use the timezone of the desktop computer to adjust the time of a dive. Again, especially for people that travel to dive, this makes no sense and so we try really hard not
to do that. We only do it for older UWatec dive computers that don't support reading the UTC offset. Right now we are adding support for a new brand of dive computer that uses a UTC-ish time, and we are not able to decode what the UTC offset setting was for the dive (no word from the manufacturer), so we had to add a "UTC offset" adjustment after Download, and before Importing to accommodate it. But this is the first brand we've had to do that for as we have been able to get the "right" time for everything else.
We'll try to get the feature mentioned in a previous post done sooner (adjust "Start time" by X hours and Y minutes for multiple dives). That seems like the best way to accommodate your particular use case and is a more general method for everyone. As you've noted, it will essentially do what you need, just one step *after* Downloading as opposed to *during* the Download process.
Let us know if that won't work for you,