Converting Dates Format into the Time Format - excel

Question

I have two dates in cell "A1" (Start Date) and "B1" (End Date). I change the date to get the real figures between the dates using SUMIFS formula.
There is Date Column in the Sheet with Time.
So what i want is that whenever the "A1" (Start Date) changes its time should always be initial like 00:00:00 and when i change the date for "B1" (End Date) it should always be like 23:59:59 is there any way to achieve this using VBA.
Your help will be appreciated.
If Not Intersect(Target, Range("A1:B1")) Is Nothing Then
With Range("A1")
.Value = Now()
.NumberFormat = "mm/dd/yyyy h:mm:ss AM/PM"
End With
With Range("B1")
.Value = Now()
.NumberFormat = "mm/dd/yyyy h:mm:ss AM/PM"
End With
End If

Answer 1

Numeric time 23:59:59 is not equal to 0.9999999, it is 0.999988425925926
But use native date/time handling for this and avoid smart numbers:
Range("A1").Value = DateValue(Range("A1").Value)
Range("B1").Value = DateValue(Range("B1").Value) + TimeSerial(23, 59, 59)

Answer 2

I think this could work, if i understood your question correct.
It will make sure A1 is integer and make B1 integer then add 0.99999.
If Not Intersect(Target, Range("A1:B1")) Is Nothing Then
Range("A1").value = cint(range("A1").value)
Range("B1").value = cint(range("B1").value) + 0.99999999
End if
Just make sure the cells is set up to display as date and time.

Answer 3

If I understand you correctly, you don't need to bother with the time portion of the dates or go throught the mechanics of trying to change it.
You just need to properly construct your SUMIFS formula.
Range to sum: sumRange
Dates in your table: myDates
When you enter a date in A1 or B1, it will be entered as if it were at the beginning of the day.
If your dates in myDates are just dates, you can use the formula:
=SUMIFS(sumRange,myDates,">=" & startDate,myDates,"<=" & endDate)
However, if your dates in myDates also include a time, then you can modify the formula slightly:
=SUMIFS(sumRange,myDates,">=" & startDate,myDates,"<" & endDate+1)
And you can use the 2nd formula in either instance
Edit: I just noticed you posting your formula as a comment
I assume you have entered JUST the date (no time) in I1 and J1
Your formula as posted will work UNLESS C:C includes times with the dates. If it does include times with the dates, then try:
=SUMIFS(D:D,E:E,"<>Pending for Payment",B:B,H2,C:C,">="&$I$1,C:C,"<"&$J$1+1)
By adding 1 to the date, we move it to the start of the day after; so we change the <= to just < and encompass the full range of dates you desire.