Google Spreadsheet having problem in Time Stamp Formula

[Syed Faizan Ali]

Hello Every one,

I need help, actually i put the time stamp formula (=IF(I11="Activity Finish",IF(J11="",NOW(),J11),"")) but it some times doesnt show correct date.
For example : i have posted activity finish on 20 march but time stamp showing 23 march .
Some Cells are working properly but some time it jumps to 2 days ahead in time stamp colum .. it must be some malfunction in formula.
Can any help me to reslove this probelm or give me another formula to add time stamp without using of script in google sheets.

Thanks