不如这样：

library(dplyr) 
library(tidyr)
d <- tibble::tribble(
  ~"User ID",   ~"2012-01-01",  ~"2012-02-01",  ~"2012-02-01", 
"Cell 1",   NA, 2,  NA,
"Cell 3",   1,  NA, 5)
d %>% 
  pivot_longer(-1, names_to="date", values_to = "vals") %>%
  na.omit() %>% 
  mutate(date = lubridate::ymd(date)) %>% 
  group_by(`User ID`) %>% 
  summarise(first = first(date), 
            last = last(date)) %>% 
  mutate(diff = last - first)
#> # A tibble: 2 × 4
#>   `User ID` first      last       diff   
#>   <chr>     <date>     <date>     <drtn> 
#> 1 Cell 1    2012-02-01 2012-02-01  0 days
#> 2 Cell 3    2012-01-01 2012-02-01 31 days

由reprex package（v2.0.1）于2022年12月13日创建
这里有一个基本的R方式（尽管使用了lubridate），它更符合你最初的想法：

d <- tibble::tribble(
  ~"User ID",   ~"2012-01-01",  ~"2012-02-01",  ~"2012-02-01", 
"Cell 1",   NA, 2,  NA,
"Cell 3",   1,  NA, 5)
d <- tibble::tribble(
  ~"User ID",   ~"2012-01-01",  ~"2012-02-01",  ~"2012-02-01", 
  "Cell 1", NA, 2,  NA,
  "Cell 3", 1,  NA, 5)

mind <- apply(d[,-1], 1, function(x)
  colnames(d[,-1])[min(which(!is.na(x)))])
maxd <- apply(d[,-1], 1, function(x)
  colnames(d[,-1])[max(which(!is.na(x)))])

d$min_date <- lubridate::ymd(mind)
d$max_date <- lubridate::ymd(maxd)
d$diff <- d$max_date - d$min_date

d
#> # A tibble: 2 × 7
#>   `User ID` `2012-01-01` `2012-02-01` `2012-02-01` min_date   max_date   diff   
#>   <chr>            <dbl>        <dbl>        <dbl> <date>     <date>     <drtn> 
#> 1 Cell 1              NA            2           NA 2012-02-01 2012-02-01  0 days
#> 2 Cell 3               1           NA            5 2012-01-01 2012-02-01 31 days

由reprex package（v2.0.1）于2022年12月13日创建

下面是一个tidyverse选项：
注意你使用的是非语法名称，而且第3列和第4列的名称相同。这在R中不起作用：

library(dplyr)
library(tidyr)

df %>% 
  mutate(across(-c(User, ID), ~case_when(!is.na(.) ~ cur_column()), .names = 'new_{col}')) %>%
  unite(non_null, starts_with('new'), na.rm = TRUE, sep = ' ') %>% 
  mutate(non_null = sub(" .*", "", non_null))

User ID X2012.01.01 X2012.02.01 X2012.02.01.1    non_null
1 Cell  1          NA           2            NA X2012.02.01
2 Cell  3           1          NA             5 X2012.01.01