I want to search for issues closed since 10am yesterday. startOfDay("-1d") only gets me from 8am... how do I tack on the last couple of hours?
Presumably you have configured your working day to start at 8am in the main JIRA configuration and likely configured JIRA to use a limited number of hours per working day as illustrated in section Configuring Time Tracking Settings within Configuring Time Tracking as well. So depending on your actual configuration there might be two solutions:
Good luck!