'At' and 'In' to Indicate Time

To show Time
1) 'At 'shows exact point of time
   e.g. Rohit reports at 7.00 A.M.
2) 'In' For long duration :
    e.g. in a day, in a month, in a year
     Shrkant will complete the work in next      
    10 hours.
3) 'within' any time in given duration
     e.g .
     They will wind it up within a week.
4) 'For' Duration of time :
     e.g.
     We waited 10 hours for them.
5) 'Since' Exact point of time in the past:
      e.g.
      Virat has been playing in Indian team since 2001.(Exact year/ Day/Date/Time)