A solution would be to backup and using the DEV version,
github.com/getk2/k2

Just chiming in to say that I have the same error - just updated to Joomla 3.73 yesterday. Articles for my site are only added on the back end, and on occasion the saves throw this:
DateTime::__construct(): Failed to parse time string (-1-11-29 18:00:00) at position 5 (-): Double timezone specification

I'm running K2 v2.7.1 with PHP version 7.0.20

Hopefully the article saves are the only bug.

Hello,

Did you try the DEV version (please backup first).

I don`t understand - what dev version? Where is it?

Try the link in the post :)

I have the same problem. Went to GitHub. No idea how to download or use anything on that page. Will a stable release fixing this bug?

For those unfamiliar with GitHub here a small guide:

1. Backup your site!
2. Create a test environment. The DEV version of K2 means the version can be unstable. Maybe this bug is solved, but some other issues can occur.
3. Go to GitHub ( github.com/getk2/k2 ) and click on the green Clone or download button. In the popup screen choose Download ZIP. You now have an install package (k2-master.zip) which you can install on your test site
4. Test K2 thoroughly. Not just this one feature, but all the essential functions you need on your site. If you come across bugs, check the issue list on GitHub . Report new issues on GitHub (note: you need an account for that)
5. If you are completely sure all is working fine, install the package on your live site. (you did make a backup before installing, didn't you?)
6. Test your live site thoroughly!
7. If this issue is not resolved or some other nasty issue comes up, then report this and hope the K2 developer team fixes this very soon. For the time being use the workaround I posted here.

Thanks for the guide Steven. I've known what GitHub was for years, but never knew quite how to use it, so this is a great hand up.

However, back to topic, this major issue is now 2 months old and still not patched. My client has reported that he tried the date change workaround, but it did not work. I'll try the code edit that Reaction posted and hope this will fix it.

Hi,
We have the same issue but we have it only a week.
Please advise.

regards,
Usu

The same problem.
There is an error if I add tags to the article.