Update the time in a tab every time the tab opens

reactive_time.R

library(shiny)

ui <- fluidPage(
  tabsetPanel(
    id = "maintabs",
    tabPanel(
      "tab1",
      "Current time is",
      textOutput("time_out", inline = TRUE)
    ),
    tabPanel(
      "tab2",
      "Tab2"
    )
  )
)

server <- function(input, output) {
  output$time_out <- renderText({
    input$maintabs
    format(Sys.time(), "%a %b %d %Y %X %Z")
  })
}

shinyApp(ui = ui, server = server)