Keep milk (or milk alternative) for a day without a fridge

I would invest in a small thermos bottle, about the size of what you need for one day. They are not only designed to keep hot food hot, they can also keep cold food cold. Choose a size that will be as full as possible when you start, it will keep better.

This is what the small B&B we‘re currently staying at supplies to their guests. If it’s good for an early morning tea, it’ll be good for afternoon coffee as well:

If you want to go all the way, you can pre-chill the container, then fill it with well-chilled milk from your home fridge. You could even freeze some milk as ice cubes, if you find that it doesn’t stay cold enough until the end of your work day or your last coffee break. Store the thermos away from heat sources and not in warm sunlight, of course.

Note that you also have the “two-hours in the danger zone” buffer and that “unsafe” doesn’t automatically mean “spoiled”. Especially when the milk is still quite cold and only shortly after the two-hour window the risk for a healthy adult should be small. No recommendation, just a thought.

Switching to milk alternatives won’t change anything where food safety is concerned.

Your best bet for longevity is UHT milk - in individual portions.
It's the same stuff you get in hotel rooms.

^{Pic from Amazon, anonymised.}
Though it doesn't taste the same as 'real' milk it's virtually indestructible, almost inert, & will survive unopened & unrefrigerated for 6 - 9 months.

As soon as it's opened, you have to treat it just like real milk, so large cartons would not be suitable.

There are dozens of brands - this was just one that Google found near the top of the list - Amazon UK - UHT MILK PORTIONS 1 x 120 portions (10ML)

In a similar situation in a previous job we successfully used an old camping trick: Wrap the bottle containing the day's milk in a damp cloth, and stand it in a bowl of water, in the draught from an open window. The evaporative cooling produced that way is really quite effective. Either buy a small bottle in the way in or transport it insulated.

Another camping style approach is a cool bag with an ice pack - put your bottle of milk in an insulated bag along with ice. You can either buy ice packs and freeze them, or 2/3 fill a drinks bottle with water and freeze that. Either way its effective enough that you might need to prevent the ice pack touching the container of milk to avoid freezing some milk

As an alternative to the (very good) vacuum bottle suggestions, Insulated lunchboxes with an ice-pack are great. My kids use them for school and the ice-pack is often still partially frozen at the end of the day, even when kept outside in hot weather.

As a bonus, you can put your other snacks/lunch in there to keep cool as well.

I don't know if this is possible for you at work but it hasn't been mentioned as an option: you can use a mini fridge to keep your milk cool / cold.

Here's an image of one

(I left the image out on purpose, because I'm not trying to advertise a specific product.)

I'm not affiliated with the manufacturer or seller, but I do have one of these fridges. I find it works well for small loads and is easy to carry if there's a need.

First of all, it's not air that makes the milk spoil but microbes that fall in and grow there. So keeping it cool is one thing, the other thing is not getting the microbes in there.

I have milk in my office outside the fridge up to about 20 °C over the working day without problems (I do have a fridge, though where I put it if I won't finish the package same day). The milk packs here come with screw caps, so I close them again immediately and compared to the cut open packages we had before this makes the milk last much longer.
I've also kept milk etc. on the outside window sill of the office when it's cooler outside.

My experience with UHT milk is: it does taste somewhat different from microfiltrated/ESL/"fresh" milk but there also seem to be differences - I've never understood people talking about revolting taste until I once had a revolting tasting UHT milk (French brand bought in Italy). Message is: it may be worth while shopping a bit around and trying whether you find UHT milk that is to your taste for the coffee. Fun fact: there are also people who like UHT milk better than "fresh" milk - e.g. for my father fresh milk always had an association of being spoilt, he said because their milk was spoilt/beginning to spoil so often when he was a kid.

While 1 l is the standard milk package size here, milk is also sold in smaller packages (I've seen 500, 250, 200 ml). There is also evaporated milk. The variety we have here is without added sugar, so unrefrigerated it lasts only marginally longer than normal milk. But you may be able to get it in small (e.g. 150 ml) packages even if you cannot find small milk packages.

Another solution (with UHT in normal packages) would be to pool up with some coworkers and speedily use up one package after the other instead of having n open milk packages spoiling in parallel. Most of the groups I've been in so far had this approach (even though we had a fridge in every place).

There are car cool boxes that double as tiny refrigerator - would that be a solution?

Insulated Stainless steel double-walled vacuum bottles, like these. Make sure to pick one with a wide mouth so that you can use a bottle brush to clean it. Just keep it out of direct sun light. Keeps cold up to 24 hours, hot up to 12 hours. Many brands, colors, sizes, configurations. No waste, reusable, will last for years. It gets really hot here in Texas and these will still have ice at the end of the day. Of course, the milk needs to be cold when you fill the bottle.

Use powdered milk designed to be added directly to coffee or tea. This will remove the hassle of pre-mixing your dried milk with water to form milk.

I keep a can of Coffee Mate around for this purpose. These are quite often termed as whiteners rather than powdered milk.

For coffee, it is like having heavy milk or cream added. To use, you spoon one spoonful of powder into your hot drink and stir.

I've taken tetrapacks of almond milk on week-long backcountry camping trips. It doesn't taste like cow's milk, but it's far enough from the uncanny valley that it doesn't ruin the coffee, it just gives it a different flavour. Unopened they last like UHT milk and once opened they're good for at least a couple of days.

I would say almond milk might be best. It doesn’t have the exact same taste as normal milk, but it is healthier as well!

Not technically an answer to the question but a solution nonetheless...

Wean yourself off milk.

I once couldn't drink coffee without cream (I used too much), saying "no sugar is tolerable, but if there's no milk, there's no coffee."

I became frustrated over spoiled cream and wasted money. Purchasing creamer in a quart size, I couldn't use it all before well past the expiration date. The smaller pint was generally gone before it went bad, but the price was significantly higher. More annoying (shouldn't have let it bother me, but it did) was the fact that pints always seemed to have dates further in the future than quarts.

So I began using less and less cream in my coffee, and now I don't mind it black. It took a couple months of being more mindful with the dose.

The bonus is that I now consume less fat (and once a year when I go to the doctor for a checkup, I needn't abstain from coffee prior to blood work).

I still splurge now and then with a bit of almond milk (or at a coffee shop I may still get a latte), but being able to drink black coffee is far more convenient, less mess, fewer utensils to wash, less money, less fuss for friends when I'm a guest.

Most important of all, it's a tiny bit less fat every morning in my diet. Every little bit helps.

Cheers!

Nestle has a product creatively called "Coffee and Milk" which is a pre-mix of sweetened condensed milk and coffee. Available in tins or in tubes, its absolutely nothing like coffee.

Since there's nothing to chill, there's no need for a fridge. Downside is the drink is boiling hot. This stuff is also the taste of tramping/hiking trips for me.

There are other Sweetened Condensed milk products which may be more accessible, but they tend to come in cans for baking purposes. If you like the taste then its a viable alternative. And some of them recommend refrigeration after opening, so check the label.

When I lived in China, the milk was sold in what could easily be described as a "juice box" and it was never refrigerated. Before moving there I was under the impression that milk always needed to be chilled, and this is simply not the case. As some of the answers above have mentioned it can stay at room temp. so long as it is unopened for up to 6 months (there are a few variables). However, depending on the container and the milk you are using (what % of M.F) it can stay opened for up to 8. I would have no issue, even then, tossing it into a hot cup of coffee.

Based on Stephen M. Webbs suggestion and cbeleites insights I experimented with a almond and oat milk, in a clean glass container. There seem to be no problems in keeping the milk from going off during the day. Personally i like oat-milk better than almond-milk. During the past weeks i have brought about 120 ml of oat milk with me daily without any adverse health effects or any funny changes in taste.

The glass container is a 130 ml glass bottle, easy to clean and dishwasher safe. (It was originally bought for storing breast milk.) This has benefits over a vaccuum flask in that 1) it is smaller and therefore easier to fit in a laptop bag, 2) it is easy to keep clean even for a disorganised person like me who leaves it in the bag overnight sometimes and finally 3) luke warm milk does not cool my coffee too much.

I may decide to store the bottle in a can-cooler or in an insulated lunchbox/bag, but it really does not seem neccesary.

Thanks everyone for this huge response to this particular 1st world problem.