Was the white wire from the switch joined to the other white before? It doesn't sound correct.
Tell us all the wires and the connections in the switch boxes you worked on.

Let's see.

Before there were two black wires, one top and one bottom on the light switch. All the white wires in the "box" were joined by a wire nut (maybe 4 total). None were going to/from the switch.

The new switch had a white wire attached. I only could make it work by adding that white wire to the bundle of white wires in the box (making a total of 5? in the nut).

Do you need more info?

THX

Sounds like you probably connected the timer right, though the fact that you apparently plowed blindly into the job and doped it out by trial and error kind of scares me. That kind of work can have disastrous results. Wasn't the timer packed with an instruction sheet?

The "upside down" switches might be 3-way switches, and their mates have since gotten thrown in the other direction. If they don't say "ON" and "OFF," they're probably 3-way.

If no white were connected to the switch before NO whites should be connected to the switch now.

Tell us ALL the wires in the box and what they are connected to now and what they were connected to before if you remember. Sounds to me like you have two devices in series.

See attached link - the manual for my timer.

The timer had 4 'built in' cables. Like 3 inches long off the back of the device. Black, white, red and green.

The old timer had two black cables and the green. I assume one of the black is what the instructions called 'load' and the other 'main'.

When I first installed the timer, I wire nutted black from timer with top black from old switch. Wire nutted red from timer to black #2 from old switch. Green was easy. At first I capped white off alone and it was not used, but the timer didn't work.

I read the instructions...Note the following:
b) Connect the Neutral wires to the White wire fromTimer. Often, the
neutral wire can be found in the back of the wiring box bound in a
wire nut.'

I then tried to find the white wire that was paired with the wires I was using. After finding it, I separated it from the bunch and connected it to white wire from switch. No luck.

Then I re-read and it says 'neutral wireS'. Plural. So sure enough I connected white from switch to all of the white wires in the box in the nut and bam, it worked.

Seems to be working fine, been about a month.

So the questions now are:

Did I do something wrong - especially when Joed say If no whites were connected before, none should now.

-and-

Could my work on that switch have reversed the polarity on switches elsewhere in the room? I don't THINK the timer is reversed. It's digital and it says off when off and on when on. I also don't think that either of the ones that are messed up are 3-way.

Thanks for your help and patience.

Jonathan

Here is a link that might be useful: Instruction manual for timer

Should I just try reversing the black wires on the two switches that are reversed? Assume that it's been like that since I moved in and just didn't notice?

Should I just try reversing the black wires on the two switches that are reversed?

Won't make any difference. Look at the switches and if the on is at the bottom and off at the top just loosen the switch and rotate till on is at the top, this is done without taking any wires off.

The OP states:
"Today, I noticed that two switches that share the same circuit (but are unrelated) are now reversed so that down is on and up is off."

If that is the only "issue" then what is the problem? If the two switches in question are single pole, the only way to create this scenario is to physically remove the switch and install it upside down.
If they're 3-way switches, it doesn't matter what position they're in as long as everything works.

Where are the white wires that are paired the blacks on the switch connected?
In this case there should be a white attached to the timer. I missed that you were installing a timer instead of a switch. Most timers do require a neutral.

Are these two swiches that are upside down now part of a three way switch setup? Are they in the same box as the timer?

Thanks for everyone's responses. I finally had a second and opened the face plate on the two switches. Turns out they were just upside down. I guess sometimes (usually?) it's the simple answer.

I had suspected that but you said they were unrelated to the other switch you worked on so I thought you never touched them.