DateTime.Rmd

---
title: "Dates and times"
output: html_notebook
---

# Dates and Times - builtin function

ISO 8601

as.Date()

library(anytime)
anytime() - recognises US data convention not british

Jan 1st 1970 is day zero

scale_x_date(date_breaks = "10 years", date_labels = "%Y")

Sys.Date()

as.POSIXlt() - list output
as.POSIXct() - seconds since Jan 1st 1970

POSIX - Portable Operating System Interface

##timezones
UTC - Coordinated Universal Time/Local

tz

# lubridate

specific function
ymd() - family corresponding to different orders
dmy_hms() - also handles datetime
parse_date_time() - general function - can pass a string of orders

make_date() - specify date from components
make_datetime()

## extracting parts of dates
year()
month()
day()
hour()
min()
second()
wday()
yday()
tz()

leap_year()
am()
pm()
dst()
quarter()
semester()

## Rounding

Useful in similar situations to extraction, but preserves the context of the unit
round_date()
ceiling_date()
floor_date()

#Arithmetic with datetimes
difftime()
now()
today()

## time spans
### period - human intuition of time
days()
### duration - stopwatch view of time
ddays()

synodic month
Saros

#### sequences of dates
today() + 1:10*days
%m+%
%m-%
add_with_rollback()

### Intervals (3rd option)
best choice when you have start and end
%--%
interval()
int_start()
int_end()
int_length()
as.period()
as.duration()
%within%
int_overlaps()

# Other stuff
## Time zones
Sys.timezone()
IANA timezones - Arthur David Olson
OlsonNames()
force_tz() - same clock time, different timezone
with_tz() - view same time in different timezone

## Times without dates
library(hms)

## Speed
library(fasttime)
fastPOSIXct()

lubridate::fast_strptime()

write_csv() knows how to handle output in ISO format
stamp()

compare processing time:
library(microbenchmark)

microbenchmark()