Fields on the lead/contact level are available through {{lead.Value}} and on the account level {{company.Value}}. So if you create start time and end time on the account level, all associated leads/contacts would be able to use the values.
But if you don't need it on the account level, but on a global level (start time and end time is the same for all leads/contacts in the system) you could just manually put it in...
Or if you have 3 different start times and end times, you could also create three SF campaigns. and add leads/contacts to those campaigns, and have different texts in the emails per campaign.
You could also create the fields in SF on the user level (for the sales rep himself). and on the lead/contact level. In SF you can create a macro to automatically copy the value to to leads the sales rep owns, wheb he changes his time value.