Unix timestamp to datetime string
JavaScript (ES6), 65 bytes

n=&gt;'101010.1010'.replace(i=/\d/g,x=&gt;new Date(n).toJSON()[i=x-~i])


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How?

We initialize the pointer \$i\$ to a non-numeric value (coerced to \$0\$) and then add alternately \$2\$ and \$1\$ to it to pick the relevant characters from the ISO-8601 conversion of the input timestamp.

yyyy-mm-ddThh:mm:ss.sssZ
  ^^ ^^ ^^ ^^ ^^




JavaScript (ES6), 66 bytes

n=&gt;'235689.BCEF'.replace(/\w/g,x=&gt;new Date(n).toJSON()[+`0x${x}`])


Try it online!

How?

Once the input timestamp is converted in ISO-8601 format, all required characters can be accessed with a single hexadecimal digit.

yyyy-mm-ddThh:mm:ss.sssZ
  ↓↓ ↓↓ ↓↓ ↓↓ ↓↓
0123456789ABCDEF