Changing the date colour when the date has past?

Hi, what is the function/formula to have a cell's text change colour when the date entered into that cell has past?

for example i have a sheet that has a list of staff names and the dates they completed mandatory training. each staff member has completed the training on different dates and therefore their date to renew the training all occur at different times. When the date has past and they are then overdue, id like the cell where that date appears to change colour to notify me at a glance that they are due for renewal. 

Thanks in advance!