Thanks for the video. I was genuinely puzzled when Casio stated +-30 sec per month for these watches. Looks like it is just a min-max threshold and the real values are much smaller. Quartz is way better in terms of precision for inexpensive watches and even for many expensive ones.

-6 secs lost a month is not an annoying imagine -6 secs times 4 months -24 secs late you can adjust the time again. The important is you're not always adjusting the time you can adjust in 1 minute advance so that you will never be late at your meeting or work.

Nice data. Just check mine, then research on CASIO accuracy, and stumble on your video. My f-91w lost about 3-4s per month. Just realize it, because alarm on phone not synchronized with my watch. Best watch for the prices.

We have now rated 9 Casio F91-W watches over several months. We find these 10-day rates: +1.18, +1.68, -1.40, +0.15, +0.47, -0.83, -0.05, -3.36, -1.16. These rates changed a bit with more data when i compare with earlier comment. These are the number of seconds gained or lost per 10 days. Not counting the best (0.05) and the worst (3.36) the average is 0.98 sec/10 days. We use exclusively now the 10 cell phone picture method to get the error to a tenth of a second on each measurement. We also learned to get early results we cannot assume the first setting was to 0.0 exactly. Thus set it to the best possible, then take 10 pics and record the errors to the tenth, then incorporate that offset in the rate computation. At some point we should streamline this article on the procedure with what we have learned. On the other hand, if you can wait a month or so, that initial error washes out. Keep in mind we are trying to get the rate accurate enough that the time can be figured accurately after several months of not setting.

Well, those casios are amazing. Mine (original one) is about +6s per 10 days, and I actually find it accurate, but these ones are amazing. My casio A-158WA is about +2s per 10 days

Thank you for the wonderful video. I have a question though: what was the average temperature during the test? My Casio is running quite a bit faster than the ones in your test and I'm wondering whether it's because it's winter now. Thanks!

David, the video is great. Thanks a lot. One question though: did you find any pattern on the accuracy differences (1.2 to 0.2) depending on the batches? Was it just a random series of watches and it just happened? In other words, is there a way to make sure you get a model from the "most accurate" batch? :)

Great info thank you ,i wish to know most accurate watch evermade

Temperature also has a lot to play in this style of watch (something to do with the quartz crystal?). They are designed to run optimally at just below body temperature (e.g 'worn') around 30 degrees...

David just FYI "A" is a letter (of the alphabet) not a number! Only joking of course (English sense of humour you see, though we're losing some of it what with Brexit & all). What a useful video & a glowing endorsement of this iconic watch. I'm off to sell my Rolex Oyster Perpetual Yachtmaster, buy another boat with the proceeds and get myself a Casio. Joking again, if only. Fair winds David...

Thanks for doing this! I always assumed that f-91ws gained time, but I see now that some lose time.

Accuracy very important

My f91w lost 1sec in 3 months. I could not believe it. Now after three years it will lose 2 secs per week. Fantastic watch for money paid.

David, $10. ?? Wow. Try Amazon for the iconic F91-W for a price of $6.75 and with Amazon Prime delivery is free

my W-800H watch runs months and only 1 sec difference to the atomic clock. weird thing is i have a genuine AQ-180W, and a replica of f-91W. the cheap replica is more accurate than the mentioned expensiver Casio. disappointment from a Casio, and surprising from a chinese replica. im on selling that casio now.