As an idiom, the past {timeperiod} is practically always used to mean "the period of time that began {timeperiod} ago and ending now." So, for me writing this right now at 2021-04-08T09:27:00-04:00 :

• "the past day" = 2021-04-07T09:27:00-04:00 to 2021-04-08T09:27:00-04:00
• "the past hour" = 2021-04-08T08:27:00-04:00 to 2021-04-08T09:27:00-04:00
• "the past year" = 2020-04-08T09:27:00-04:00 to 2021-04-08T09:27:00-04:00

For day in particular, "the past day" can also mean "yesterday", somewhat informally.

As @randomhead noted in his answer, the reason this language was used rather than just saying "yesterday" is because the time that the survey was published is not the same as the time the survey is filled out.

As an idiom, the past {timeperiod} is practically always used to mean "the period of time that began {timeperiod} ago and ending now." So, for me writing this right now at 2021-04-08T05:27:00Z:

• "the past day" = 2021-04-07T05:27:00Z to 2021-04-08T05:27:00Z
• "the past hour" = 2021-04-08T04:27:00Z to 2021-04-08T05:27:00Z
• "the past year" = 2020-04-08T05:27:00Z to 2021-04-08T05:27:00Z

For day in particular, "the past day" can also mean "yesterday", somewhat informally.

As @randomhead noted in his answer, the reason this language was used rather than just saying "yesterday" is because the time that the survey was published is not the same as the time the survey is filled out.