find the difference between two columns in specific rows
for i in range (99,len(df),200):
    try:
       df1=df.loc[i+200,'acctimestamp'] - df.loc[i+200,'gyrtimestamp']
       print(df1)
    except: 
       print('End')


Is what you're looking for? Step is 200. So i will take 299 -> 499 -> 699 etc.., and you're taking only the rows between i ( 299 ) and i + 200 ( 499 ) and so on.