Update Cursor skipping last row?

Question

I have created a python function within field calculator to calculate the time difference between a timestamp (yyyy-MM-dd HH:mm:ss) in a row and the row after (the interval field in seconds). I have used a update cursor with field calculator to do this, however I am receiving null values for both the first and last row.

The first is correctly missed due to the function being reliant on having a value set by the previous row.

I am unsure as to why the last row is being skipped?

The first row of the attribute is formatted as:

The last:

I have had this issue on several other update cursor functions including those on simple single row condition statements.

Am I missing a statement to close the update cursor loop?

My code is:

import arcpy, time, datetime
from time import strftime
from datetime import timedelta, datetime
from arcpy import da

def FindTime(table,date,interval):

  firstRow = True

  with arcpy.da.UpdateCursor(table, ["interval", "date"]) as cursor:

    for row in cursor:
        gap2 = row[1]

        if firstRow == True:
            gap1 = gap2
            firstRow = False
            continue
        timedelta = gap2 - gap1
        row[0] = timedelta.days * 24 * 3600 + timedelta.seconds
        gap1 = gap2         
        cursor.updateRow(row)

*Updated from below comment's solution:

import arcpy
def FindTime(fc, datefield, daydiff_field):

    all_dates = [i[0] for i in     arcpy.da.SearchCursor(fc,datefield)]

    diff = [(d1-d0).seconds for d0,d1 in zip(all_dates, all_dates[1:])]
    givediff = iter(diff)

    with arcpy.da.UpdateCursor(fc, daydiff_field) as cursor:
        next(cursor) #uncomment this line if you want to calculate row2 diff as row2-row1, =first row no diff
        for row in cursor:
            try:
                row[0] = next(givediff) #Fetch diffs until list is empty...
                cursor.updateRow(row)
            except StopIteration: #...then break the cursor
                break

Answer 1

If your field is type date, code below should work. Last line is not calculated since there is no row after. Or do you want to calculate for example second rows diff as second row-first row?

import arcpy

fc = 'somedates'
datefield = 'date123'
daydiff_field = 'seconddiff_long'

all_dates = [i[0] for i in arcpy.da.SearchCursor(fc,datefield)]

diff = [(d1-d0).seconds for d0,d1 in zip(all_dates, all_dates[1:])]
givediff = iter(diff)

with arcpy.da.UpdateCursor(fc, daydiff_field) as cursor:
#next(cursor) #uncomment this line if you want to calculate row2 diff as row2-row1, =first row no diff
    for row in cursor:
        try:
            row[0] = next(givediff) #Fetch diffs until list is empty...
            cursor.updateRow(row)
        except StopIteration: #...then break the cursor
            break