Convert a datetime Text to DateTime object

I would use the datetime function and split your datetime string so you have the necessary properties. That function will return a value of DateTime.

In the past I needed to do a much similar conversion - here's the expression rule I used:

with(
  local!dateTimeValueArray: split(substitute(substitute(substitute( ri!string,  " ", ":" ), "-", ":"), ".", ":"), ":"),
  if(
    rule!APN_isEmpty(local!dateTimeValueArray), 
    null(),
    datetime(
      local!dateTimeValueArray[1],
      local!dateTimeValueArray[2],
      local!dateTimeValueArray[3],
      index(local!dateTimeValueArray, 4, 0),
      index(local!dateTimeValueArray, 5, 0),
      index(local!dateTimeValueArray, 6, 0),
      index(local!dateTimeValueArray, 7, 0)
    )
  )
)

Use the value function to parse text date into a date.  see docs.appian.com/.../fnc_text_value.html