What, like this? You should specify the example better in your question so that people can answer it.
library(tidyverse)
library(lubridate)
NewData <- tibble::tribble(
~gvkey, ~date, ~a, ~b,
1L, "01/01/2000", 15624000, 10011531920,
1L, "02/01/2000", 18480000, 10592724720,
1L, "03/01/2000", 16968000, 10160611743,
2L, "01/01/2000", 9375333997, 10011531920,
2L, "02/01/2000", 9943536057, 10592724720,
2L, "03/01/2000", 9470034340, 10160611743,
3L, "01/01/2000", 441665123.3, 10011531920,
3L, "02/01/2000", 412041243.1, 10592724720,
3L, "03/01/2000", 417427403.1, 10160611743
) %>%
mutate(date = dmy(date))
NewData %>%
group_by(date) %>%
summarise(a = sum(a), b = sum(b))
# A tibble: 3 x 3
date a b
<chr> <dbl> <dbl>
1 01/01/2000 9832623120. 30034595760
2 02/01/2000 10374057300. 31778174160
3 03/01/2000 9904429743. 30481835229