Um, I have to ask, why are you trying to change this?
The reason it’s not working for you, is that this is not a client setting. As @cmcannady states, his action command is the correct syntax for modifying a setting, but settings are stored at [Software\BigFix\EnterpriseClient\Settings\Client], not at [Software\BigFix\EnterpriseClient\GlobalOptions].
What you’re modifying is the client’s understanding of the system boot time, which should change every time the machine reboots. I supposed this exists in that location because the ‘boot time of operating system’ inspector may be expensive, and it is convenient for performance reasons to cache the result in that location.