US roaming.

[Terryscell]

I will visit the USA for 15 days, and need to exactly match the 15 day roaming plan to those dates.

The website states that plan activates "when processed", and "will be processed within 24 hours".

Can I set the dates in advance, or apply the night before travel, and hope for the best?

[Terryscell]

Thanks, everyone; all great and useful responses. Sorry I can't accept all as Solution.

[DennyCrane · Accepted solution]

This 

To add to this, I would suggest adding funds to your account at home, and then once you arrive in the US you can purchase the add-on. I only say this because I wouldn't recommend using public wi-fi to make any credit card purchases. Or if you have a VPN, even better.

[slusagm]

you cannot buy ahead and set a date to start 

Last time, I bought it, it started immediately. Just make sure you turn roaming data on your phone and make sure phone connects to T-Mobile or AT&T

[hTideGnow]

hi @Terryscell 

for US roaming, the 15 days count down as well as the roaming service start instantly.  I would buy it when you start the trip, maybe in the airport or when you are close to the broader if you drive

[hairbag1]

@Terryscell 

when you arrive in US, go to nearest wifi area and log in to your account...then buy that add-on. You'll have it for 360 hours. Make sure you have volte enabled for best talk-ability in US.

added...

when in US, make sure you're connected to either AT&T or T-Mobile...no others will work.

The add-on usually starts working as soon as you successfully buy it..no waiting major period.