R replace minimum date values per group - r

Question

I have a df with observations on different groups for a year. However, date of the first observation can differ slightly per group (generally within the first days of the year). I'm planning to show these groups in one lineplot and I want them all to start on "2021-01-01".
How can I recode my date variable as such that the first occurrence (min(Date)?) per group is set to "2021-01-01"?
Here is a small subset, with the X, Y, Z having different starting dates. Thanks!
structure(list(Date = structure(c(18637, 18644, 18651, 18658,
18665, 18672, 18679, 18686, 18693, 18700, 18707, 18714, 18721,
18728, 18735, 18636, 18643, 18651, 18656, 18665, 18672, 18676,
18686, 18693, 18700, 18707, 18714, 18720, 18727, 18735, 18635,
18643, 18649, 18658, 18662, 18670, 18677, 18684, 18692, 18700,
18707, 18713, 18718, 18728, 18735), class = "Date"), Maand = structure(c(1L,
1L, 1L, 1L, 2L, 2L, 2L, 2L, 3L, 3L, 3L, 3L, 4L, 4L, 4L, 1L, 1L,
1L, 1L, 2L, 2L, 2L, 2L, 3L, 3L, 3L, 3L, 4L, 4L, 4L, 1L, 1L, 1L,
1L, 2L, 2L, 2L, 2L, 3L, 3L, 3L, 3L, 4L, 4L, 4L), .Label = c("jan",
"feb", "mrt", "apr", "mei", "jun", "jul", "aug", "sep", "okt",
"nov", "dec"), class = c("ordered", "factor")), UPV2 = c(339L,
69L, 59L, 48L, 77L, 95L, 54L, 61L, 99L, 95L, 67L, 71L, 54L, 98L,
98L, 8L, 6L, 11L, 7L, 15L, 7L, 5L, 4L, 22L, 13L, 4L, 5L, 14L,
14L, 7L, 6L, 7L, 8L, 13L, 2L, 9L, 9L, 13L, 4L, 9L, 8L, 8L, 4L,
14L, 4L), VAR = structure(c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L,
2L, 2L, 2L, 2L, 2L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L,
3L, 3L, 3L, 3L), .Label = c("X", "Y", "Z"), class = "factor")), row.names = c(NA,
-45L), groups = structure(list(VAR = structure(1:3, .Label = c("X",
"Y", "Z"), class = "factor"), .rows = structure(list(1:15, 16:30,
31:45), ptype = integer(0), class = c("vctrs_list_of", "vctrs_vctr",
"list"))), row.names = c(NA, -3L), class = c("tbl_df", "tbl",
"data.frame"), .drop = TRUE), class = c("grouped_df", "tbl_df",
"tbl", "data.frame"))

Answer 1

This solution with dplyr (and lubridate) will target every occurrence of the minimum Date for each group, and replace it with your common starting date of DEFAULT_DATE. As of my recent revision, it will also update the custom month abbreviation in Maand.
library(dplyr)
library(lubridate)
# ...
# Code to generate your data.frame "df".
# ...
DEFAULT_DATE <- as.Date("2021-01-01")
df <- df %>%
group_by(VAR) %>%
mutate(# Update the custom month abbreviation for every "min(Date)" in each group.
Maand = if_else(Date == min(Date),
# Pick out the corresponding level of the factor.
ordered(levels(Maand)[month(DEFAULT_DATE)], levels = levels(Maand)),
Maand),
# Replace every "min(Date)" in each group.
Date = if_else(Date == min(Date), DEFAULT_DATE, Date)) %>%
ungroup()
Keep in mind that most of the complication here arises from your custom abbreviations for month names, as factorized (with ordering) in the Maand column.
Fortunately, my revised solution addresses this challenge. If a new group "A" were added to the mix, and its earliest Date were 2021-03-07, then its Maand would be your custom abbreviation for "March", which in this case is "mrt". When applying my transformation, that date would be updated to DEFAULT_DATE, which in this case is 2021-01-01. Furthermore, the mutate() would also ensure that the Maand is updated (here to "jan"): to the level of the factor (here the 1st level) that corresponds to the month of the DEFAULT_DATE (here the 1st month of the year).