Does Appian has any default function to get the Max/ latest date?

I believe dates in that format will sort lexically consistently (i.e. a standard "alphabetical" sorting will sort the dates correctly, assuming they're TEXT and with no other conversion necessary), in which case I'd expect the "max()" function might work for you.

Thanks for the suggestion Mike. I tried to use the max function and for ( List of Text String: 2 items "2018-12-04T19:24:44" "2017-09-26T20:41:35") and this is what I got -2.017093e+13

My mistake, max() does expect a numeric value.  The place I've used it for finding the max date from a list is being called on an array of dateTime value.

The attached alternative code sample (which i've actually tested this time) will demonstrate how you can manually sort your text array (using officially supported functionality), and using a the specified pagingInfo value, select just the first result automatically (which should be the highest).  Let me know if you have any questions.

with(
  local!array: {
    "2018-12-04T19:24:44",
    "2017-09-26T20:41:35"
  },
  
  todatasubset(
    a!forEach(
      local!array,
      {
        text: fv!item
      }
    ),
    a!pagingInfo(
      startIndex: 1,
      batchSize: 1,
      sort: a!sortInfo(
        field: "text",
        ascending: false()
      )
    )
  ).data.text
)

The attached alternative code sample (which i've actually tested this time) will demonstrate how you can manually sort your text array (using officially supported functionality), and using a the specified pagingInfo value, select just the first result automatically (which should be the highest).  Let me know if you have any questions.

with(
  local!array: {
    "2018-12-04T19:24:44",
    "2017-09-26T20:41:35"
  },
  
  todatasubset(
    a!forEach(
      local!array,
      {
        text: fv!item
      }
    ),
    a!pagingInfo(
      startIndex: 1,
      batchSize: 1,
      sort: a!sortInfo(
        field: "text",
        ascending: false()
      )
    )
  ).data.text
)

Thank you. It worked.