I, like many others, am really surprised that Zoho Creator doesn't cater for a time field (hh:mm) to capture time spent on certain tasks. So I've opted to use a decimal field and ask people to round up to the nearest half an hour.  

I would like to restrict the allowable numbers after the decimal point so that they can only enter .5 or .0 and nothing else. If they try to enter anything other than a 5 or a 0 after the decimal point then a message will display - "please enter time to the nearest half an hour". 

Can someone help me with this please? The field I'm talking about is "time_spent" in this form:  https://creator.zoho.com/lindsay.hodgson/wellbeing-review/#Form:Wellbeing_review 

Many thanks, 

Lindsay