In order to achieve reproducible builds, timestamps used in the U-Boot build process have to be set to a fixed value.
This is done using the SOURCE_DATE_EPOCH environment variable which specifies the number of seconds since 1970-01-01T00:00:00Z.
To build the sandbox with 2023-01-01T00:00:00Z as timestamp we can use:
make sandbox_defconfig SOURCE_DATE_EPOCH=1672531200 make
This date is shown when we launch U-Boot:
./u-boot -T U-Boot 2023.01 (Jan 01 2023 - 00:00:00 +0000)
The same effect can be obtained with buildman using the -r flag.