Young adult time-travelling book with the tag-line "what if you could slip in and out of time"

Question

Basically, what the above says; there's this book that I remember reading a long time ago in a library that I did read a few times and remember most of the plot to, but I can't remember the darn title or the names of some of the characters. For context here, I would've been reading it sometime around the age of 13-14, back in around 2007.

The book itself is, again, a young-adult book about time travel with that tagline on the cover. As for the main theme, it stars a boy who is friends with a professor who basically invents a time machine that resembles a calculator. No, it isn't Strange Attractors - rather than it being a story about someone trying to steal the time machine and the like, the book is more of a collection of short-stories revolving around the boy and some parts of his life, not necessarily all about time travel but not necessarily in chronological order either.

Main highlights of the story that I can remember:

• Boy has a sister that seems to be in ill health. At one point she's talked about as if she's passed away and the boy expresses some sadness about not being around in her final days, while later on in the book there is a short story about him spending time with her on what's hinted at to be one of her final days. Like I said before, time travel is kind of implied here and not outright talked about and the latter scene is implied to have happened before earlier ones.
• The calculator-like time machine eventually runs out of power near the end of the book. Boy, through tinkering, discovers a small but heavy metal cube that seems to be the power source - I remember distinctly that the cube was described as "seeming to make up the majority of the weight of the whole device," though I'm not sure if that was the exact wording. Boy brings it to an electronics shop to see if there was a way to charge it, the guy behind the counter identifies it and confirms "yeah no, I can't charge this, and I didn't even know it could be recharged - this thing normally has enough power stored in it to power a city for years, and I might be in trouble if it gets out that I even know what this is." In the end, shop guy keeps the cube.
• In one of the scenes involving the boy's sister, they're talking about a book she's reading currently that, in a very meta-like fashion, she describes as a collection of short stories that are all connected in one way or another but it isn't clear immediately how.

That's all I can remember about the book. Which sucks because I do remember reading it a lot, but I can't even find any hints about what the book could be. Hopefully someone here has an idea?

Answer 1

I think this is Out of Time (1990) by John Marsden, republished by Tor Teen in 2005.

The cover shows the tag "What if you could slide in - and out - of time?"

The time machine looks like a calculator:

He thought at first that he was looking at nothing more than a mathematical calculator. Mr. Woodforde had assembled all the pieces into a black case that resembled either a calculator or a remote control for a television. There were four panels of buttons, two aerials and a screen, but they were all on a small enough scale to be fitted into a hand-sized case.

Finding the battery:

He prised open the cover of the little battery compartment and pulled out the battery. Bright, bright silver, it was like no other power source he had ever seen. It was tiny; the size of a gambling die. And the shape of one. But it was surprisingly heavy, weighing more than all the rest of the machine.

It has a lot of power, and can't be recharged:

"I'll be," he muttered. "Talkative little fellow aren't you? Look," he said, "you don't recharge them. They're not like a battery. But on the other hand, you can't run them out either. Well, not that anyone's been able to so far. If you've been running your torch off it, then I guarantee you a few millennia of bright light yet. Matter of fact, you could keep this base lit up for way past your lifetime, just on one of these little beauties."