Default Appian JSON Format Not Compatible With External System in Integration

The JSON you constructed is anyway not valid. If you try to pass it in a!fromJson, it will give you an error. Rather, use the below code snippet. I have tried to simply it also for you. 

a!toJson(
  value: {
    a!map(
      startPublicationDate: text(ri!startPublicationDate, "yyyy-mm-dd"),
      endPublicationdate: text(ri!endPublicationDate, "yyyy-mm-dd")
    )
  }
)

The JSON you constructed is anyway not valid. If you try to pass it in a!fromJson, it will give you an error. Rather, use the below code snippet. I have tried to simply it also for you. 

a!toJson(
  value: {
    a!map(
      startPublicationDate: text(ri!startPublicationDate, "yyyy-mm-dd"),
      endPublicationdate: text(ri!endPublicationDate, "yyyy-mm-dd")
    )
  }
)

Some information about JSON - 

1. In JSON, square brackets represent an array. So if you want to add them, just wrap your data in {} on Appian because Appian uses {} for an array. 
2. Appian's JSON doesn't work differently than the rest of the internet. JSONs are universally standard and you should have no problem trying to use a!toJson and a!fromJson as long as your map is correct. 

I have tried that code before and it gives the same error as in the posted screenshot. However, creating an expression and calling it in the integration body works:

Expression (ABC_SearchParams):

a!localVariables(
  local!startDt: todate("05/09/2021"),
  local!jsonArray: append(
    {},
    {
      "startPublicationDate": text(local!startDt, "YYYY-MM-DD"),
      "endPublicationDate": text(local!startDt, "YYYY-MM-DD")
    }
  ),
  local!jsonArray
)

Integration Body (Content Body: JSON):

rule!ABC_SearchParams()

Result:

There shouldn't be a reason why one is working and the other one is not. There must be a problem with the body you are passing. 

the working code uses "endPublicationDate", while the failing code uses "endPublicationdate". Do you spot the difference?

I had not spotted the difference, the second body with the concat() function works now that I have fixed it. Thank you!