Now
You cannot get the current time in LIGO but you can get the starting time of the current block :
let today : timestamp = Tezos.now
Arithmetics
In LIGO, timestamps can be added to integers :
let today : timestamp = Tezos.now
let one_day : int = 86_400
let in_24_hrs : timestamp = today + one_day
let some_date : timestamp = ("2000-01-01t10:10:10Z" : timestamp)
let one_day_later : timestamp = some_date + one_day
let in_24_hrs : timestamp = today - one_day
Comparison
You can compare timestamps using the same comparison operators applying to numbers.
let not_tomorrow : bool = (Tezos.now = in_24_hrs)
Your mission
1- Compute the constant timestamp eta as our estimated time from arrival in 6 days from now.