Using lubridate to get dates/times from continuous increments - r

Question

I am trying to use lubridate to process the results of a differential equation solved using ode. My simulation begins on a certain date (01-01-2021) and is on the order of days (a one unit-time increase is equal to a one day calendar time increase). How can I use lubridate to process a continuous double of time since simulation start?
For ex, I want to go from the left column to the right column:
ODE time
Calendar Time
0.0
01-01-2021 00:00
0.5
01-01-2021 12:00
1.0
01-02-2021 00:00
etc...
Thank you

Answer 1

I am not fully sure I understand your question. But from your example it appears you want to create timesteps. When I understand it correctly, a "one unit" is a adding 24 hours, while the half day is adding 12 hours. Your data frame example suggest you want to have this in a dataframe/tibble.
With {lubridate} you can "coerce" datetimestamps. There are some handy time formatting functions. From a character you can go to a timestamp.
For example
# create dataframe/tibble of ODE and Calendar times
mydata <- tribble(
~ODE_time, ~Calendar_Time
,0.0 , "01-01-2021 00:00"
,0.5 , "01-01-2021 12:00"
,1.0 , "01-02-2021 00:00"
,1.5 , "01-02-2021 12:00"
)
mydata <- mydata %>%
mutate(time = lubridate::mdy_hm(Calendar_Time))
In your case, I use the mdy_hm() function to make a timestamp (dttm) object.
I assign it to the time variable/column so you can check the presentation in R/RStudio.
What I get from your question is that you want to create a sequence of timestamps.
Here you can use the seq() function and work with the time offset, in your case 12 hours (or half a day). I limit the length out to 10 ... you can obviously define longer sequences or determine your end day (i.e. to parameter of seq())
date_time_seq <- seq( from = lubridate::mdy_hm("01-01-2021 00:00")
,length.out = 10,
,by = "12 hours")
This gives you a sequence of timestamps
date_time_seq
[1] "2021-01-01 00:00:00 UTC" "2021-01-01 12:00:00 UTC" "2021-01-02 00:00:00 UTC"
[4] "2021-01-02 12:00:00 UTC" "2021-01-03 00:00:00 UTC" "2021-01-03 12:00:00 UTC"
[7] "2021-01-04 00:00:00 UTC" "2021-01-04 12:00:00 UTC" "2021-01-05 00:00:00 UTC"
[10] "2021-01-05 12:00:00 UTC"
The syntax allows you to add various "steps" and you can use increments of different time units, e.g. mins, hours, days, weeks, etc.
This timestep vector you can operate in your dataframe/tibble and perform your other operations.
Good luck!

Answer 2

You could directly add the number of seconds to the start date:
ODETime <- seq(0,10,by=0.5)
calendarTime <- as.POSIXct("2021-01-01 00:00") + ODETime * 86400
calendarTime
[1] "2021-01-01 00:00:00 CET" "2021-01-01 12:00:00 CET" "2021-01-02 00:00:00 CET"
[4] "2021-01-02 12:00:00 CET" "2021-01-03 00:00:00 CET" "2021-01-03 12:00:00 CET"
[7] "2021-01-04 00:00:00 CET" "2021-01-04 12:00:00 CET" "2021-01-05 00:00:00 CET"
[10] "2021-01-05 12:00:00 CET" "2021-01-06 00:00:00 CET" "2021-01-06 12:00:00 CET"
[13] "2021-01-07 00:00:00 CET" "2021-01-07 12:00:00 CET" "2021-01-08 00:00:00 CET"
[16] "2021-01-08 12:00:00 CET" "2021-01-09 00:00:00 CET" "2021-01-09 12:00:00 CET"
[19] "2021-01-10 00:00:00 CET" "2021-01-10 12:00:00 CET" "2021-01-11 00:00:00 CET"
or with lubridate:
as.POSIXct("2021-01-01 00:00") + lubridate::period(24,'hour') * ODETime
[1] "2021-01-01 00:00:00 CET" "2021-01-01 12:00:00 CET" "2021-01-02 00:00:00 CET"
[4] "2021-01-02 12:00:00 CET" "2021-01-03 00:00:00 CET" "2021-01-03 12:00:00 CET"
[7] "2021-01-04 00:00:00 CET" "2021-01-04 12:00:00 CET" "2021-01-05 00:00:00 CET"
[10] "2021-01-05 12:00:00 CET" "2021-01-06 00:00:00 CET" "2021-01-06 12:00:00 CET"
[13] "2021-01-07 00:00:00 CET" "2021-01-07 12:00:00 CET" "2021-01-08 00:00:00 CET"
[16] "2021-01-08 12:00:00 CET" "2021-01-09 00:00:00 CET" "2021-01-09 12:00:00 CET"
[19] "2021-01-10 00:00:00 CET" "2021-01-10 12:00:00 CET" "2021-01-11 00:00:00 CET"