For me time blindness shows up in 3-4 main ways (but I'm not sure if the fourth is a time blindness thing or a normal human thing)
- Poor perception of how long things take, usually in the optimistic direction, (Oh that will only take me ten minutes!) leading to not allowing enough time for things. Conversely sometimes it's pessimistic which can cause me to procrastinate on a task that will actually only take 5 minutes.
- Going into hyperfocus during a particularly engaging task which means that I lose all sense of the outside world/reality, so I look up and it's about 3 hours later than I realised.
- Everything has its own timeline/time track and they are all loose and don't seem to be fixed to the main one. So unless I keep notes of when it was that I last worked on something, or I can link it to something else (Oh yes, when I spoke to Sharon she was wearing her Christmas earrings, so it was probably December) I will have no idea - once I go past yesterday, two days feels about the same as up to 3-4 weeks, a couple of weeks feels about the same as up to several months, a few months feels about the same as 2-5 years ish. Anything older than this is just "ages ago" or linked to a life period e.g. "when I was at school/before DS2 was born" I have also been known to say "That was yesterday" to DH and he looks confused and says "No, it was this morning/three days ago".
- Not really being aware of what date/time/day it is without looking it up. (I don't know if this is innate for anybody, is it?)
Forgetting about a meeting is not quite the same skill, it's more organisation or working memory I think. Organisation is the skill of keeping a diary, planner, choosing activities based on how they fit with others, keeping everything needed accessible and organised so it can be found when needed, etc.
Working memory is more about keeping necessary information in mind - this is impaired with ADHD and there's nothing much you can do about it, except externalise it - for example, having a whiteboard with the current focuses or today's task list or important info visible on it, minimising distractions which might take up space in WM and "push out" other important tasks.
Thinking about problems which may occur and accommodations or things which would help with each item.
Poor perception of how long something takes - this could cause issues with accurately reporting or estimating time taken/time needed for tasks. Time tracking software can help here if the job is mostly computer based, so that the person can look back and see how long it took a previous time - it does need to be used correctly, labelled etc. There might need to be some time allocated at the end of the day for example so that the person can go through and tag all entries. There may be some kind of non computer based time tracker possible where the person can just click a button to indicate they are task switching and then at the end of the day they can go through all the entries and label them (but this might cause issues if they forget to click, which is possible).
What may also help is an estimate being given from someone with better time perception, if this is appropriate (e.g. I think time taken is often taken into account in project management, so there will be rough numbers for these things somewhere). In terms of physical presence at meetings, not online meetings, if the person works on a large site then it might be worth them walking around and physically timing how long it takes them to walk between various spaces, including a toilet break. Or the simple thing is as someone else suggested, they should aim to be 5-10 minutes early for each meeting, and/or be given explicit instruction about what they can do if they are early for a meeting. Being early can provoke a sense of anxiety if you aren't sure what to do and are prone to being drawn into accidental hyperfocus e.g. I will just look at my phone while I wait for people. That's why the aim is often to be exactly on time (which, of course, has a high risk of being late!)
Interrupting hyperfocus can be achieved with regular check-ins with the real world e.g. some kind of IM software, reminders, beeping/speaking clock etc. Again there will be some accessibility software, or there might be something built into a PC, which can automatically speak out reminders, or for a physical office there might be some kind of smart alarm or a smartphone app which will do the same thing. The person can activate settings on their phone to automatically silence/louden the phone when entering or leaving certain areas. The downside of this is that hyperfocus (which isn't really something you can switch on and off at will) can produce really good work and be extremely frustrating to be interrupted; interruptions might also be ignored or brushed aside. If you're trying to help someone then it might be an idea to have a conversation about how best to balance keeping track of real time while allowing the hyperfocus to happen sometimes too.
Really good tracking and particularly visual timelines can really help integrate things into a consolidated timeline, rather than a list of dates which is basically just a meaningless jumble of numbers. What can also help here is some kind of software which automatically orders tasks based on a combination of how frequently they should be done, when they were last done, how important they are, whether they are overdue, whether they block other tasks or other people, any important deadlines, etc. I use this for example for housework, and there are apps which will do this for contacts - I'm sure that there must be applicable examples in many jobs. This is veering into organisation and prioritisation, which are also hard for people with ADHD, but I think that this aspect of time management is probably a huge part of what is behind this. A person with ADHD is going to expend an enormous amount of energy if they have to work all these things out manually, which depletes their capacity to perform at other things, so any tools at all which help with this are going to reduce that load and let them use their energy where their strengths lie instead.
Lastly knowing what day/date/time it is - lots of visible clocks, calendars, etc. I find I'm more aware of where I am in the week if I have specific and distinct differences between the days e.g. when I was teaching EFL and I'd be at company A on Monday, company B and C on Tuesday, kids' classes at my workplace on Wednesday, evening class, Thursday off, Friday group class, planning time etc. And because I was constantly using the date by marking off who was present and writing the date I was much more aware of where we are in a month. So if this is something which can be implemented it will help someone be more aware of where they are in time, which might help with the consolidating timelines thing.