How do I set the time of day and date on the acurite 1099RX

https://acu-rite.com/support/product-literature/

Battery compartment contains reset button. Press. Will need to re-enter time and date.

https://www.acurite.com/kbase/weather-stations/02031RM_Weather_Station.html

page 6 and 7

Intelli-Time
®
clock and calendar are pre-programmed to the correct time and
date, and update automatically for Daylight Saving Time. All you need to do is
select your time zone using the "ZONE" button


https://www.acurite.com/media/manuals/13020-13026-instructions.pdf

I have no idea what this 1099RX is.

Set the Time & Date
The clock and calendar are utilized to time stamp history records and other data, so it is important to set the time and date soon after you power on the display unit.
Set the Time
1. Press the "up arrow" or "down arrow" buttons until "SET CLOCK?" is displayed on the Selectable Category part of the display unit.
2. Press the "SET" button to set the clock.
3. Press the "up arrow" or "down arrow" to adjust the hour.
Note the "AM" and "PM" indicators.
4. Press the "SET" button to confirm hour
selection.
5. Press the "up arrow" or "down arrow" to adjust minutes.
6. Press the "SET" button to confirm minute
selection.
The clock is now set.

https://www.acurite.com/media/manuals/01010W-instructions.pdf

BACK OF DISPLAY UNIT
1. A-B-C Switch
ID code that must match 5-in-1 sensor’s A-B-C switch to ensure units synchronize.

If this is for the AcuRite weather station, please contact customer support at: http://www.acurite.com/support

You can download the manual here http://www.acurite.com/kbase/00621_Weather_Station.html
Set the Time, Date & Units Press the "SET" button, located on the back of the display unit, to enter SET MODE. Once in set mode, the preference you are currently setting will blink on the display.
To adjust the currently selected (flashing) item, press and release the "up" or "down" buttons (press and HOLD to fast adjust).
To save your adjustments, press and release the "SET" button again to adjust the next preference. The preference set order is as follows:
TIME ZONE ( PST MST CST EST AST HAST AKST) DST (Daylight Saving Time ON or OFF)*
CLOCK HOUR
CLOCK MINUTE
CALENDAR MONTH
CALENDAR DATE
CALENDAR YEAR
TEMPERATURE SCALE ( ?F or ?C) BAROMETRIC PRESSURE SCALE (inHg or hPa)
*If you live in an area that observes DST, DST should be set to ON, even if it is not currently Daylight Saving Time.
You will automatically exit SET MODE if no buttons are pressed for 30 seconds. Enter setup mode at any time by pressing the "SET" button.

http://www.acurite.com/advancedsearch/result/?q=1099RX

There is no reset button, installing new batteries causes the sensor to reset itself.

Try removing and re-installing the batteries.

Good luck,
Steve

I have misplaced the user manual for the
Acurite L5C0668TXACU RITE Wireless Thermometer Model L5C0668TX. Where can I get another?

Before attempting to set the time, let the outdoor transmitter sync with the indoor receiver. Once that's done, you'll be able to successfully set and retain the date and time.