Merge pull request #534 from prometheus/dates

Document new date/time related functions.

content/docs/querying/functions.md

@@ -6,14 +6,19 @@ sort_rank: 3
 
 # Functions
 
+Some functions have default arguments, e.g. `year(v=vector(time())
+instant-vector)`. This means that there is one argument `v` which is an instant
+vector, which if not provided it will default to the value of the expression
+`vector(time())`.
+
 ## `abs()`
 
-`abs(v vector)` returns the input vector with all sample values converted to
+`abs(v instant-vector)` returns the input vector with all sample values converted to
 their absolute value.
 
 ## `absent()`
 
-`absent(v vector)` returns an empty vector if the vector passed to it has any
+`absent(v instant-vector)` returns an empty vector if the vector passed to it has any
 elements and a 1-element vector with the value 1 if the vector passed to it has
 no elements.
 
@@ -63,6 +68,22 @@ vector as a scalar. This is in contrast to the `count()`
 always returns a vector (an empty one if the input vector is empty) and allows
 grouping by labels via a `by` clause.
 
+## `day_of_month()`
+
+`day_of_month(v=vector(time()) instant-vector)` returns the day of the month
+for each of the given times in UTC. Returned values are from 1 to 31.
+
+## `day_of_week()`
+
+`day_of_week(v=vector(time()) instant-vector)` returns the day of the week for
+each of the given times in UTC. Returned values are from 0 to 6, where 0 means
+Sunday etc.
+
+## `days_in_month()`
+
+`days_in_month(v=vector(time()) instant-vector)` returns number of days in the
+month for each of the given times in UTC. Returned values are from 28 to 31.
+
 ## `delta()`
 
 `delta(v range-vector)` calculates the difference between the
@@ -162,6 +183,11 @@ more trends in the data is considered. Both `sf` and `tf` must be between 0 and
 
 `holt_winters` should only be used with gauges.
 
+## `hour()`
+
+`hour(v=vector(time()) instant-vector)` returns the hour of the day
+for each of the given times in UTC. Returned values are from 0 to 23.
+
 ## `idelta()`
 
 `idelta(v range-vector)`
@@ -251,6 +277,12 @@ The special cases are equivalent to those in `ln`.
 `log10(v instant-vector)` calculates the decimal logarithm for all elements in `v`.
 The special cases are equivalent to those in `ln`.
 
+## `month()`
+
+`month(v=vector(time()) instant-vector)` returns the month of the year for each
+of the given times in UTC. Returned values are from 1 to 12, where 1 means
+January etc.
+
 ## `predict_linear()`
 
 `predict_linear(v range-vector, t scalar)` predicts the value of time series
@@ -325,6 +357,12 @@ expression is to be evaluated.
 
 `vector(s scalar)` returns the scalar `s` as a vector with no labels.
 
+## `year()`
+
+`year(v=vector(time()) instant-vector)` returns the year
+for each of the given times in UTC.
+
+
 ## `<aggregation>_over_time()`
 
 The following functions allow aggregating each series of a given range vector