Increase precision of xls datetimes when coerced to character

[jennybc]

Fixes #430 read_xls rounding date times when col_types="text"

[jennybc]

@jimhester This "fixes" the problem but I am not entirely satisfied because

• I don't have a good handle on why the precision was previously so low (?) on the xls side, nor how it's currently set on the xlsx side.
• I feel uncertain about how many digits of agreement I should expect between xlsx and xls in the test.

General context, in case it's not clear: until the day when we can translate xls(x) date time formats to R date time formats, datetimes will be coerced to character as if they were just regular doubles.

Do you have any advice re: setting precision and/or testing?

---

devtools::load_all(here::here())
#> Loading readxl
xlsx <- read_excel(test_sheet("texty-dates-xlsx.xlsx"), col_types = "text")
xls <- read_excel(test_sheet("texty-dates-xls.xls"), col_types = "text")

xlsx
#> # A tibble: 2 x 1
#>   a                 
#>   <chr>             
#> 1 31117.541666666672
#> 2 31117.558009259261
xls
#> # A tibble: 2 x 1
#>   a            
#>   <chr>        
#> 1 31117.5416667
#> 2 31117.5580093

Created on 2018-03-15 by the reprex package (v0.2.0).

[jimhester]

Why can't we do this now?

translate xls(x) date time formats to R date time formats

[jimhester]

This needs to be something like std::numeric_limits<double>::digits10 + 2 (which ends up being 17 for 64bit doubles) to ensure full precision. See the proposal linked at https://stackoverflow.com/questions/554063/how-do-i-print-a-double-value-with-full-precision-using-cout#comment29144568_554134 for details why.

[jennybc]

OK thanks. Do you think I should pre-emptively do same on the xlsx side, so they are both doing the "right" thing and doing the same thing? Disregard: just remembered that there is no coercion on the xlsx side -- the serial date is already a string.

Re: converting time format strings, our impression is that this is doable but it's also not a tiny piece of work. The plan is to use or build on this: https://github.com/WizardMac/TimeFormatStrings, which would have application across multiple packages.

[jimhester]

Alright, I apparently misunderstood how the dates are represented in xls format, LGTM!