If the StartDate reflects when the event happened, then the StartDate should be stored in _time. You need to set this up using props.conf on the indexer (unless you are using a heavy forwarder to collect the data). Splunk only uses the time that it indexed the data when it can't find any other timestamp automatically and nothing is configured in props.conf to tell it how to parse. You should read Configure timestamps and fix your data.
There is no way to easily select the StartDate range, as all searches are based on _time. However, if you search over "All Time" or a sufficiently large time period (last 90 days, perhaps?), you can filter based on StartDate. For this to work, StartDate will need to be a Unix-formatted time. It might work something like this.
| eval Start = strptime(StartDate,"%m-%d-%Y")
| where Start >=relative_time(now(),"-7d@d")
This will select events that have a StartDate within the last week. For it to work, the timerange picker will have to select all events that might have a StartDate with an appropriate time.