Compute Fractional Day of Year from POSIXct

Calculates the fractional day of year from a POSIXct datetime object. The fractional day is zero-indexed, starting at 0 for January 1st at midnight and ending at approximately 365.958 for December 31st at 23:00 in a non-leap year (or 366.958 in a leap year).

Usage

fractional_day_of_year(datetime)

Arguments

datetime

A POSIXct object or vector of POSIXct objects representing date-time values. Must have a timezone attribute.

Value

A numeric vector of the same length as datetime, containing fractional day of year values. January 1st at midnight corresponds to 0, and each hour adds approximately 0.04167 (1/24) to the value.

Details

The function computes the time difference in hours between the input datetime and midnight on January 1st of the same year, then divides by 24 to obtain fractional days. The calculation accounts for leap years automatically.

Examples

dates <- seq(
  from = as.POSIXct("2024-01-01 00:00:00", tz = "UTC"),
  to = as.POSIXct("2024-01-02 00:00:00", tz = "UTC"),
  by = "6 hours"
)
fractional_day_of_year(dates) # Returns 0.00, 0.25, 0.50, 0.75, 1.00
#> [1] 0.00 0.25 0.50 0.75 1.00

# End of year
dt_end <- as.POSIXct("2024-12-31 23:00:00", tz = "UTC")
fractional_day_of_year(dt_end) # Returns ~365.958
#> [1] 365.9583