Description Usage Arguments Examples
Add periods to datetime objects. Periods track the change in the "clock time" between two civil times. They are measured in common civil time units: years, months, days, hours, minutes, and seconds.
Arithmetic operations with multiple period units (years, months
etc) are applied in decreasing size order, from year to second. Thus
time_add(x, months = 1, days = 3)
first adds 1 to x
and then 3
days.
Generally period arithmetic is undefined due to the irregular nature of civil time and complexities with DST transitions. 'timechange' allows for a refined control of what happens when an addition of irregular periods (years, months, days) results in "unclear" date.
Let's start with an example. What happens when you add "1 month 3 days" to
"20000131 01:02:03"? 'timechange' operates by applying larger
periods first. First months are added1 + 1 = February
which results in
nonexistent time of 20000231 01:02:03
. Here the roll_month
adjustment kicks in:
skip
 no adjustment is done to the simple arithmetic operations (the
gap is skipped as if it's not there. Thus, 20000131 01:02:03 + 1 month + 3 days
is equivalent to 20000101 01:02:03 + 1 month + 31 days + 3 days
resulting in 20000305 01:02:03
.
NA
 if any of the intermediate additions result in nonexistent dates
NA
is produced. This is how arithmetic in lubridate
operates.
boundary
 if an intermediate computation falls in a gap, the date is
adjusted to the next valid time. Thus, 20000131 01:02:03 + month = 20000301 00:00:00
.
next
 is like boundary
but preserves the smaller units. Thus,
20000131 01:02:03 + 1 month = 20000301 01:02:03
.
prev
 is like next
but instead of rolling forward to the first day of
the month, it rolls back to the last valid day of the previous month. Thus,
20000131 01:02:03 + 1 month = 20000228 01:02:03
. This is the default.
1 2 3 4 5 6 7  time_add(time, periods = NULL, years = NULL, months = NULL,
weeks = NULL, days = NULL, hours = NULL, minutes = NULL,
seconds = NULL, roll_month = "last", roll_dst = "first")
time_subtract(time, periods = NULL, years = NULL, months = NULL,
weeks = NULL, days = NULL, hours = NULL, minutes = NULL,
seconds = NULL, roll_month = "last", roll_dst = "last")

time 
datetime object 
periods 
string of units to add/subtract (not yet implemented) or a
named list of the form 
years, months, weeks, days, hours, minutes, seconds 
Units to be added to

roll_month 
controls how addition of months and years behaves when standard arithmetic rules exceed limits of the resulting date's month. See "Details" for the description of possible values. 
roll_dst 
controls how to adjust the updated time if it falls within a DST transition intervals. See the "Details". 
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45  # Addition
## Month gap
x < as.POSIXct("20000131 01:02:03", tz = "America/Chicago")
time_add(x, months = 1, roll_month = "first")
time_add(x, months = 1, roll_month = "last")
time_add(x, months = 1, roll_month = "boundary")
time_add(x, months = 1, roll_month = "skip")
time_add(x, months = 1, roll_month = "NA")
time_add(x, months = 1, days = 3, roll_month = "first")
time_add(x, months = 1, days = 3, roll_month = "last")
time_add(x, months = 1, days = 3, roll_month = "boundary")
time_add(x, months = 1, days = 3, roll_month = "skip")
time_add(x, months = 1, days = 3, roll_month = "NA")
## DST gap
x < as.POSIXlt("20100314 01:02:03", tz = "America/Chicago")
time_add(x, hours = 1, minutes = 50, roll_dst = "first")
time_add(x, hours = 1, minutes = 50, roll_dst = "last")
time_add(x, hours = 1, minutes = 50, roll_dst = "boundary")
time_add(x, hours = 1, minutes = 50, roll_dst = "skip")
time_add(x, hours = 1, minutes = 50, roll_dst = "NA")
# SUBTRACTION
## Month gap
x < as.POSIXct("20000331 01:02:03", tz = "America/Chicago")
time_subtract(x, months = 1, roll_month = "first")
time_subtract(x, months = 1, roll_month = "last")
time_subtract(x, months = 1, roll_month = "boundary")
time_subtract(x, months = 1, roll_month = "skip")
time_subtract(x, months = 1, roll_month = "NA")
time_subtract(x, months = 1, days = 3, roll_month = "first")
time_subtract(x, months = 1, days = 3, roll_month = "last")
time_subtract(x, months = 1, days = 3, roll_month = "boundary")
time_subtract(x, months = 1, days = 3, roll_month = "skip")
time_subtract(x, months = 1, days = 3, roll_month = "NA")
## DST gap
y < as.POSIXlt("20100315 01:02:03", tz = "America/Chicago")
time_subtract(y, hours = 22, minutes = 50, roll_dst = "first")
time_subtract(y, hours = 22, minutes = 50, roll_dst = "last")
time_subtract(y, hours = 22, minutes = 50, roll_dst = "boundary")
time_subtract(y, hours = 22, minutes = 50, roll_dst = "skip")
time_subtract(y, hours = 22, minutes = 50, roll_dst = "NA")

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