df <- data.frame("one"=c(1,2,3,4),"two"=c(4,NA,2,1))
storeddate <- "Jan17-01-92"
finaldf <- df %>%
 mutate(!!storeddate := one+two) %>%
 filter(!is.na(storeddate)) #the storeddate string call requires formatting as `Jan17-01-92` as opposed to "Jan17-01-92".

finaldf <- df %>%
 mutate(!!storeddate := one+two) %>%
 filter(!is.na(!!storeddate))

df %>%
  mutate(!!storeddate := one+two) %>%
  filter(!is.na(!!as.name(storeddate)))

#   one two Jan17-01-92
# 1   1   4           5
# 2   3   2           5
# 3   4   1           5