[carl_dr]

Sounds like a great Code Golf contest.

ruby -e 'require"date";a=DateTime.new(0,1,1,0,0,0);(0..4e9).each{|d|b=(a+Rational(d,86400));c=b.strftime("%y%m%d%H%M%S");puts b.strftime("%y-%m-%d %H:%M:%S")if c==c.reverse}'

About the most inefficient anyone could make it, but I think it works!

[AkshatJ27]

Some saves: `Rational(d,86400)` -> `d/86400r`

Get rid of the variable `a` since it is used in only one place.

`DateTime.new(0,1,1,0,0,0)` -> `DateTime.new(0,1,1)`

also in newer ruby versions you can use numbered parameters to replace d with _1 and remove `|d|`

Pretty sure there might be some more which i do not see immediately :)

[carl_dr]

ruby -e 'require"date";(0..4e9).each{b=DateTime.new(0,1,1)+_1/86400r;c=b.strftime("%y%m%d%H%M%S");puts b.strftime("%y-%m-%d %H:%M:%S")if c==c.reverse}'

151 bytes (including the ruby -e, down from 174.) Thanks!

[AkshatJ27]

`ruby -e 'require"date";0.upto(4e9){b=DateTime.new(0,1,1)+_1/86400r;c=b.strftime"%y%m%d%H%M%S";p b.strftime"%y-%m-%d %H:%M:%S"if c==c.reverse}'`

142? Sorry if I broke anything, on phone.

[carl_dr]

You need to use puts to avoid quotes being added around the output - so I'll give you 145! I'm on my phone too, I'm impressed I even got something working, you've done brilliantly.

Edit: DateTime.new(0) seems to work. 141.

[AkshatJ27]

~~`b=DateTime.new(0,1,1)+_1/86400r;`-> `b=_1/86400r+DateTime.new 0`~~ oops

[carl_dr]

ruby -rdate -e'b=DateTime.new 0;loop{b+=1/86400r;c=b.strftime"%y%m%d%H%M%S";puts b.strftime"%y-%m-%d %H:%M:%S"if c==c.reverse}'

127. Script never terminates, but it does display all possibilities, so I think it still passes!

Edit: 126. ruby -rdate -e'b=DateTime.new 0;loop{b+=1/86400r;puts b.strftime"%y-%m-%d %H:%M:%S"if(c=b.strftime"%y%m%d%H%M%S")==c.reverse}'

I think I'm done for the moment, will revisit tomorrow if you can find any further savings!

[carl_dr]

That doesn't work unfortunately, you need the brackets - DateTime has to be on the left.

[PinkMilkshake]

PowerShell 7, 108 chars

  0..2e9|%{date(date 0).AddSeconds($_)-f 'yy-MM-dd HH:mm:ss'}|?{$_-match'(.)(.)-(.)(.)-(.)(.) \6\5:\4\3:\2\1'}

[lcnPylGDnU4H9OF]

Looks like it would be defeated by the dreaded leap second.

[SAI_Peregrinus]

They're going away. Resolution 4 of the CGPM 2022[1]. They didn't want to deal with having to add a negative leap second, and it looks likely they'd have to if they didn't end them.

[1] https://www.bipm.org/documents/20126/64811223/Resolutions-20...

[carl_dr]

I don't think so. It would be only year 2006 (which map to second 60) which could be a problem, and there wasn't a leap year in that year.