Use lubridate::as_datetime in get_timeseries_signature_date function

[BradThymesAtTheElRoyale]

The lubridate::as_date() function relies on the user's timezone, which causes some issues with retrieving the week of the month at the beginning or end of a month when getting the time series signature. Switching to lubridate::as_datetime() solves this issue and returns the expected week of the month. Reprex included below.

Sys.setenv(TZ = 'America/Chicago')
stringi::stri_datetime_fields(lubridate::as_date('2022-11-01'))
#>   Year Month Day Hour Minute Second Millisecond WeekOfYear WeekOfMonth
#> 1 2022    10  31   19      0      0           0         45           6
#>   DayOfYear DayOfWeek Hour12 AmPm Era
#> 1       304         2      7    2   2
stringi::stri_datetime_fields(lubridate::as_datetime('2022-11-01'))
#>   Year Month Day Hour Minute Second Millisecond WeekOfYear WeekOfMonth
#> 1 2022    11   1    0      0      0           0         45           1
#>   DayOfYear DayOfWeek Hour12 AmPm Era
#> 1       305         3      0    1   2

^{Created on 2023-05-10 with reprex v2.0.2}