Function of present perfect:

I haven't seen her

since Monday       (days of the week)

since (3^rd) March (names of the months)

since 1989            (dates)

since Christmas  (festive days)

since summer     (seasons)

since lunch          (time of meals)

since yesterday  (days in relation to now)

since the end of the 18^th century 

since I was in London (defined time, date)

ever since 

for two hours     (hours)

for three days    (days)

for four weeks   (weeks)

for five months  (months)

for six years       (years)

for ages              (long period)

for a long time   (period of 

                               time in general