<s>
MAC	B-Application
times	I-Application
are	O
pieces	O
of	O
file	B-Application
system	I-Application
metadata	O
which	O
record	O
when	O
certain	O
events	O
pertaining	O
to	O
a	O
computer	B-Operating_System
file	I-Operating_System
occurred	O
most	O
recently	O
.	O
</s>
<s>
The	O
events	O
are	O
usually	O
described	O
as	O
"	O
modification	O
"	O
(	O
the	O
data	O
in	O
the	O
file	O
was	O
modified	O
)	O
,	O
"	O
access	O
"	O
(	O
some	O
part	O
of	O
the	O
file	O
was	O
read	O
)	O
,	O
and	O
"	O
metadata	O
change	O
"	O
(	O
the	O
file	O
's	O
permissions	O
or	O
ownership	O
were	O
modified	O
)	O
,	O
although	O
the	O
acronym	O
is	O
derived	O
from	O
the	O
"	O
mtime	O
"	O
,	O
"	O
atime	O
"	O
,	O
and	O
"	O
ctime	O
"	O
structures	O
maintained	O
by	O
Unix	B-Operating_System
file	I-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
Windows	O
file	B-Application
systems	I-Application
do	O
not	O
update	O
ctime	O
when	O
a	O
file	O
's	O
metadata	O
is	O
changed	O
,	O
instead	O
using	O
the	O
field	O
to	O
record	O
the	O
time	O
when	O
a	O
file	O
was	O
first	O
created	O
,	O
known	O
as	O
"	O
creation	O
time	O
"	O
or	O
"	O
birth	O
time	O
"	O
.	O
</s>
<s>
Some	O
other	O
systems	O
also	O
record	O
birth	O
times	O
for	O
files	O
,	O
but	O
there	O
is	O
no	O
standard	O
name	O
for	O
this	O
metadata	O
;	O
ZFS	B-Application
,	O
for	O
example	O
,	O
stores	O
birth	O
time	O
in	O
a	O
field	O
called	O
"	O
crtime	O
"	O
.	O
</s>
<s>
MAC	B-Application
times	I-Application
are	O
commonly	O
used	O
in	O
computer	O
forensics	O
.	O
</s>
<s>
Because	O
most	O
file	B-Application
systems	I-Application
do	O
not	O
compare	O
data	O
written	O
to	O
a	O
file	O
with	O
what	O
is	O
already	O
there	O
,	O
if	O
a	O
program	O
overwrites	O
part	O
of	O
a	O
file	O
with	O
the	O
same	O
data	O
as	O
previously	O
existed	O
in	O
that	O
location	O
,	O
the	O
modification	O
time	O
will	O
be	O
updated	O
even	O
though	O
the	O
contents	O
did	O
not	O
technically	O
change	O
.	O
</s>
<s>
In	O
Windows	O
,	O
starting	O
with	O
Vista	B-Application
,	O
file	B-Application
access	I-Application
time	O
updating	O
is	O
disabled	O
by	O
default	O
.	O
</s>
<s>
Unix	B-Application
and	O
Windows	O
file	B-Application
systems	I-Application
interpret	O
'	O
ctime	O
 '	O
differently	O
:	O
</s>
<s>
Unix	B-Application
systems	I-Application
maintain	O
the	O
historical	O
interpretation	O
of	O
ctime	O
as	O
being	O
the	O
time	O
when	O
certain	O
file	O
metadata	O
,	O
not	O
its	O
contents	O
,	O
were	O
last	O
changed	O
,	O
such	O
as	O
the	O
file	O
's	O
permissions	O
or	O
owner	O
(	O
e.g.	O
</s>
<s>
This	O
difference	O
in	O
usage	O
can	O
lead	O
to	O
incorrect	O
presentation	O
of	O
time	O
metadata	O
when	O
a	O
file	O
created	O
on	O
a	O
Windows	O
system	O
is	O
accessed	O
on	O
a	O
Unix	B-Application
system	I-Application
and	O
vice	O
versa	O
.	O
</s>
<s>
Although	O
not	O
specified	O
by	O
POSIX	O
,	O
most	O
modern	O
Unix	B-Operating_System
file	I-Operating_System
systems	I-Operating_System
(	O
such	O
as	O
ext4	B-Application
,	O
HFS+	B-Application
,	O
ZFS	B-Application
,	O
and	O
UFS2	B-Operating_System
)	O
allow	O
to	O
store	O
the	O
creation	O
time	O
.	O
</s>
<s>
NTFS	B-Application
stores	O
both	O
the	O
creation	O
time	O
and	O
the	O
change	O
time	O
.	O
</s>
<s>
A	O
different	O
approach	O
is	O
for	O
creation	O
times	O
to	O
stand	O
for	O
when	O
the	O
file	B-Application
system	I-Application
object	O
itself	O
was	O
created	O
,	O
e.g.	O
</s>
<s>
As	O
with	O
all	O
file	B-Application
system	I-Application
metadata	O
,	O
user	O
expectations	O
about	O
MAC	B-Application
times	I-Application
can	O
be	O
violated	O
by	O
programs	O
which	O
are	O
not	O
metadata-aware	O
.	O
</s>
<s>
Some	O
file-copying	O
utilities	O
will	O
explicitly	O
set	O
MAC	B-Application
times	I-Application
of	O
the	O
new	O
copy	O
to	O
match	O
those	O
of	O
the	O
original	O
file	O
,	O
while	O
programs	O
that	O
simply	O
create	O
a	O
new	O
file	O
,	O
read	O
the	O
contents	O
of	O
the	O
original	O
,	O
and	O
write	O
that	O
data	O
into	O
the	O
new	O
copy	O
,	O
will	O
produce	O
new	O
files	O
whose	O
times	O
do	O
not	O
match	O
those	O
of	O
the	O
original	O
.	O
</s>
<s>
Windows	O
is	O
not	O
affected	O
by	O
this	O
due	O
to	O
a	O
workaround	O
feature	O
called	O
File	B-Application
System	I-Application
Tunneling	O
.	O
</s>
