Yes – once the expiry date has passed you can no longer use your survey. While you will be able to collect results on your device (this cannot be automatically disabled) you will not be able to upload them to the Data4Action server.
For example, if you bought a survey to use for one month on 05 August then you will not be able to upload any more results to the Data4Action web site after midnight on 04 September has passed.
It is very important that you keep track of your expiry dates otherwise you could end up with survey results which you cannot use. Data4Action emails you warnings prior to your survey expiry.