How can we get difference between two times ?

After.. decrypting.. your question, I understood that you want to find the difference in time between 2 date/dateTime values, here you go mate; this should help:

a!localVariables(
  local!firstDateTime: now(),
  local!secondDateTime: subtractminutes(subtracthours(now(), 4), 15),
  local!timeDifference: elapsedtime(
    local!secondDateTime,
    local!firstDateTime
  ),
  a!map(
    mintuesDifference: local!timeDifference[5],
    hoursDifference: local!timeDifference[4],
    dateDifference: elapseddays(
      local!secondDateTime,
      local!firstDateTime
    ),
    hoursWithMinutes: time(
      local!timeDifference[4],
      local!timeDifference[5]
    )
  )
)

Thank you for the answer elapseddays() function is not avaliable 

elapsedtime()... my locals' names are self-explanatory, its right there mate, I recommend reading the function description to fully understand it. 

Its a custom function, available under this plugin

community.appian.com/.../date-and-time-utilities

ah, so you don't have the plugin, my bad, here's the equivalent of the above but by using Appian built-in functions:

a!localVariables(
  local!firstDateTime: now(),
  local!secondDateTime: now() - intervalds(28, 15, 0),
  local!differenceTimeValue: timevalue(
    local!firstDateTime - local!secondDateTime
  ),
  a!map(
    mintuesDifference: minute(local!differenceTimeValue),
    hoursDifference: hour(local!differenceTimeValue),
    hoursWithMinutes: local!differenceTimeValue,
    dateDifference: days360(
      local!secondDateTime,
      local!firstDateTime
    )
  )
)

ah, so you don't have the plugin, my bad, here's the equivalent of the above but by using Appian built-in functions:

a!localVariables(
  local!firstDateTime: now(),
  local!secondDateTime: now() - intervalds(28, 15, 0),
  local!differenceTimeValue: timevalue(
    local!firstDateTime - local!secondDateTime
  ),
  a!map(
    mintuesDifference: minute(local!differenceTimeValue),
    hoursDifference: hour(local!differenceTimeValue),
    hoursWithMinutes: local!differenceTimeValue,
    dateDifference: days360(
      local!secondDateTime,
      local!firstDateTime
    )
  )
)

how can we capture only time from the user is there any specific field of we just need to give 2 drop downs for hours and minutes 

Not getting correct answer 

I don't know if this is what you are looking for, but you can use a!elapseTime and then map the result like the picture below.

working thank you

you are welcome