long story short: <span t-field="start_date" t-field-options='{"format": "%H:%M"} />

Use odoo date field

start_date = fields.Date('Date')

Instead of float

start_date: field.Float()

And in qweb reports use

<span t-field="doc.start_date"/>

For more information visit

1- http://learnopenerp.blogspot.com/2016/11/how-to-create-qweb-reports-in-openerp.html

2- http://learnopenerp.blogspot.com/2016/09/how-to-create-custom-reports-in-odoo.html