added comment from copilot

Author: douwe

R/utils.R

@@ -57,7 +57,45 @@ has_star <- function(x) {
 #' `format(as.POSIXct(x, tz=""), format="%Y-%m-%d")` to get a string with format
 #' YYYY-MM-DD.
 #'
-#  TODO: check the previous statements
+#  TODO: check the previous and following statements
+#
+# ACCORDING TO Copilot:
+# ### **Analysis**:
+# 1. **Excel Date Storage**:
+#    - Excel does **not** store dates as the number of days since January 1, 1970. Instead:
+#      - Excel stores dates as the number of days since **January 1, 1900** (for Windows systems) or **January 1, 1904** (for macOS systems).
+#      - Excel also incorrectly assumes that 1900 was a leap year, which introduces an offset of 1 day for dates before March 1, 1900.
+#
+# 2. **`read_excel` Behavior**:
+#    - When using `readxl::read_excel`, Excel dates are typically read as numeric values representing the number of days since Excel's epoch (e.g., 1900 or 1904). These values are **not automatically converted to POSIXct** by `read_excel`. The user must manually convert them.
+#
+# 3. **POSIXct Conversion**:
+#    - The description mentions converting the number to a date using `as.POSIXct(as.integer(x))`. However:
+#      - This assumes that the numeric value `x` is already in seconds since January 1, 1970, which is not the case for Excel dates.
+#      - To convert Excel dates to R's `POSIXct`, you need to account for Excel's epoch (e.g., subtract the appropriate offset for 1900 or 1904).
+#
+# 4. **Formatting**:
+#    - The description correctly states that `format(as.POSIXct(x, tz=""), format="%Y-%m-%d")` can be used to format a `POSIXct` object as a string in the `YYYY-MM-DD` format.
+#
+# ### **Corrected Description**:
+# Here’s a revised and accurate version of the description:
+# 
+# Excel stores dates as numeric values representing the number of days since
+# January 1, 1900 (Windows) or January 1, 1904 (macOS). Note that Excel's 1900
+# date system incorrectly assumes 1900 was a leap year, which introduces a
+# 1-day offset for dates before March 1, 1900.
+#
+# When read using `readxl::read_excel`, Excel dates are imported as numeric
+# values. To convert these to R's `POSIXct` class, you must account for Excel's
+# epoch. For example, subtract 25569 days (the number of days between January 1,
+# 1900, and January 1, 1970) and convert to seconds by multiplying by 86400.
+#
+# Example conversion:
+# `as.POSIXct((x - 25569) * 86400, origin = "1970-01-01", tz = "")`
+#
+# To format the date as a string in `YYYY-MM-DD` format, use:
+# `format(as.POSIXct(...), format = "%Y-%m-%d")`.
+#
 #' @return A vector of the specified atomic class
 #' @noRd
 coerce <- function(x, atomicclass) {