<s>
A	O
hash	B-Algorithm
calendar	I-Algorithm
is	O
a	O
data	O
structure	O
that	O
is	O
used	O
to	O
measure	O
the	O
passage	O
of	O
time	O
by	O
adding	O
hash	B-Algorithm
values	O
to	O
an	O
append-only	O
database	O
with	O
one	O
hash	B-Algorithm
value	O
per	O
elapsed	O
second	O
.	O
</s>
<s>
It	O
can	O
be	O
thought	O
of	O
special	O
kind	O
of	O
Merkle	B-Algorithm
or	I-Algorithm
hash	I-Algorithm
tree	I-Algorithm
,	O
with	O
the	O
property	O
that	O
at	O
any	O
given	O
moment	O
,	O
the	O
tree	O
contains	O
a	O
leaf	O
node	O
for	O
each	O
second	O
since	O
1970‑01‑01	O
00:00:00	O
UTC	O
.	O
</s>
<s>
By	O
periodically	O
publishing	O
the	O
root	O
of	O
the	O
hash-tree	O
is	O
it	O
possible	O
to	O
use	O
a	O
hash	B-Algorithm
calendar	I-Algorithm
as	O
the	O
basis	O
of	O
a	O
hash-linking	O
based	O
digital	O
timestamping	O
scheme	O
.	O
</s>
<s>
The	O
hash	B-Algorithm
calendar	I-Algorithm
construct	O
was	O
invented	O
by	O
Estonian	O
cryptographers	O
Ahto	O
Buldas	O
and	O
Mart	O
Saarepera	O
based	O
on	O
their	O
research	O
on	O
the	O
security	O
properties	O
of	O
cryptographic	B-Algorithm
hash	I-Algorithm
functions	I-Algorithm
and	O
hash-linking	O
based	O
digital	O
timestamping	O
.	O
</s>
<s>
There	O
are	O
different	O
algorithms	O
that	O
can	O
be	O
used	O
to	O
build	O
a	O
hash	B-Algorithm
calendar	I-Algorithm
and	O
extract	O
a	O
relevant	O
hash	B-Algorithm
chain	O
per	O
second	O
.	O
</s>
<s>
The	O
hash	B-Algorithm
chains	O
can	O
then	O
be	O
extracted	O
as	O
from	O
any	O
hash	B-Algorithm
tree	O
.	O
</s>
<s>
Since	O
the	O
hash	B-Algorithm
calendar	I-Algorithm
is	O
built	O
in	O
a	O
deterministic	O
manner	O
,	O
the	O
shape	O
of	O
the	O
tree	O
for	O
any	O
moment	O
can	O
be	O
reconstructed	O
knowing	O
just	O
the	O
number	O
of	O
leaf	O
nodes	O
in	O
the	O
tree	O
at	O
that	O
moment	O
,	O
which	O
is	O
one	O
more	O
than	O
the	O
number	O
of	O
seconds	O
from	O
1970‑01‑01	O
00:00:00	O
UTC	O
to	O
that	O
moment	O
.	O
</s>
<s>
Therefore	O
,	O
given	O
the	O
time	O
when	O
the	O
calendar	O
tree	O
was	O
created	O
and	O
a	O
hash	B-Algorithm
chain	O
extracted	O
from	O
it	O
,	O
the	O
time	O
value	O
corresponding	O
to	O
each	O
leaf	O
node	O
can	O
be	O
computed	O
.	O
</s>
<s>
The	O
Distributed	O
hash	B-Algorithm
calendar	I-Algorithm
is	O
a	O
distributed	O
network	O
of	O
hash	B-Algorithm
calendar	I-Algorithm
nodes	O
.	O
</s>
<s>
In	O
order	O
to	O
ensure	O
a	O
high	O
availability	O
service	O
it	O
is	O
possible	O
to	O
have	O
multiple	O
calendars	O
in	O
different	O
physical	O
locations	O
all	O
of	O
which	O
communicate	O
with	O
each	O
other	O
to	O
ensure	O
that	O
each	O
calendar	O
contains	O
identical	O
hash	B-Algorithm
values	O
.	O
</s>
<s>
A	O
five	O
node	O
hash	B-Algorithm
calendar	I-Algorithm
cluster	O
is	O
a	O
component	O
of	O
Keyless	O
Signature	O
Infrastructure	O
(	O
KSI	O
)	O
,	O
each	O
leaf	O
in	O
the	O
hash	B-Algorithm
calendar	I-Algorithm
being	O
the	O
aggregate	O
hash	B-Algorithm
value	O
of	O
a	O
globally	O
distributed	O
hash	B-Algorithm
tree	O
.	O
</s>
