How to compute the field value a time interval earlier?

[yshen]

I want to compute the change in temperature for each location in a given interval, say, 15 minutes, or 30 minutes. I figure that streamstats might capture the temperature value at the beginning of such time interval, using time_window to specify the interval length. But, however, the following example surprises me.
The temperature readings for Pleasonton are collected every 15 minutes, thus the following query:

| makeresults
| eval _raw="time_  Location    Temperature
2021-08-23T03:04:05.000-0700    Pleasonton  185
2021-08-23T03:04:20.000-0700    Pleasonton  86
2021-08-23T03:04:35.000-0700    Pleasonton  87
2021-08-23T03:04:50.000-0700    Pleasonton  89"
| multikv forceheader=1
| eval _time=strptime(time_,"%Y-%m-%dT%H:%M:%S.%3N%z")
| fields _time Location Temperature
| sort _time
| streamstats earliest(Temperature) as previous_temp earliest(_time) as previous_time by Location time_window=5m
| convert ctime(previous_time)

I’d expect the following, as with the interval 5 minutes from an event, there is no other event, but the current one.

_time	Location	Temperature	_raw	previous_temp	previous_time
2021-08-23 03:04:05	Pleasonton	185	2021-08-23T03:04:05.000-0700 Pleasonton 185	185	08/23/2021 03:04:05.000000
2021-08-23 03:04:20	Pleasonton	86	2021-08-23T03:04:20.000-0700 Pleasonton 86	86  	08/23/2021 03:04:20.000000
2021-08-23 03:04:35	Pleasonton	87	2021-08-23T03:04:35.000-0700 Pleasonton 87	87  	08/23/2021 03:04:35.000000
2021-08-23 03:04:50	Pleasonton	89	2021-08-23T03:04:50.000-0700 Pleasonton 89	89  	08/23/2021 03:04:50.000000

but this is actually what I get:

_time	Location	Temperature	_raw	previous_temp	previous_time
2021-08-23 03:04:05	Pleasonton	185	2021-08-23T03:04:05.000-0700 Pleasonton 185	185	08/23/2021 03:04:05.000000
2021-08-23 03:04:20	Pleasonton	86	2021-08-23T03:04:20.000-0700 Pleasonton 86	185	08/23/2021 03:04:05.000000
2021-08-23 03:04:35	Pleasonton	87	2021-08-23T03:04:35.000-0700 Pleasonton 87	185	08/23/2021 03:04:05.000000
2021-08-23 03:04:50	Pleasonton	89	2021-08-23T03:04:50.000-0700 Pleasonton 89	185	08/23/2021 03:04:05.000000

All taking the earliest event's temperature, which is beyond 5 minutes from any subsequent events.How can I query to get the temperature at the beginning of the time period?