count no of days

I find it helpful to create an Expression Rule in my environments that can do a null-safe "elapsed days" comparison between any 2 dates, in the manner required by your system (i.e. sometimes if the starting and ending date are the same, you'd want to return a value of "1" instead of "0", in other words doing an "end dates inclusive" value).  For example the following:

/* RULE_CalculateElapsedDays */
/* Takes a Start and End date and calculates total elapsed (i.e. deployed) days,
  inclusively -- e.g. if the start and end date are the same day,
  the total elapsed days will be 1 instead of 0. */

if(
  or(
    a!isNullOrEmpty(ri!startDate),
    a!isNullOrEmpty(ri!endDate)
  ),
  tointeger(null()),
  
  tointeger(ri!endDate - ri!startDate) + 1
)

I find it helpful to create an Expression Rule in my environments that can do a null-safe "elapsed days" comparison between any 2 dates, in the manner required by your system (i.e. sometimes if the starting and ending date are the same, you'd want to return a value of "1" instead of "0", in other words doing an "end dates inclusive" value).  For example the following:

/* RULE_CalculateElapsedDays */
/* Takes a Start and End date and calculates total elapsed (i.e. deployed) days,
  inclusively -- e.g. if the start and end date are the same day,
  the total elapsed days will be 1 instead of 0. */

if(
  or(
    a!isNullOrEmpty(ri!startDate),
    a!isNullOrEmpty(ri!endDate)
  ),
  tointeger(null()),
  
  tointeger(ri!endDate - ri!startDate) + 1
)