3 added 43 characters in body
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JavaScript ES6, 9898 130 bytes

s=>s==+s?new Date'246060'.replace(s*86400/../g,l=>':'+('0'+~~(s*=+l)%60).toTimeStringslice(-2)).split` `[0]slice(1):s.split`:`.reduce((a,b)=>+b+(+a)*60)*1/864e2864e2;f(0.5);

JavaScript ES6, 98 bytes

s=>s==+s?new Date(s*86400).toTimeString().split` `[0]:s.split`:`.reduce((a,b)=>+b+(+a)*60)*1/864e2

JavaScript ES6, 98 130 bytes

s=>s==+s?'246060'.replace(/../g,l=>':'+('0'+~~(s*=+l)%60).slice(-2)).slice(1):s.split`:`.reduce((a,b)=>+b+(+a)*60)*1/864e2;f(0.5);
2 deleted 5 characters in body
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JavaScript ES6, 10298 bytes

s=>s==+s?new Date(s*86400).toTimeString().split` `[0]:s.split`:`.reduce((a,b)=>+b+((+a)*60))*(1*1/86400)864e2

JavaScript ES6, 102 bytes

s=>s==+s?new Date(s*86400).toTimeString().split` `[0]:s.split`:`.reduce((a,b)=>+b+((+a)*60))*(1/86400)

JavaScript ES6, 98 bytes

s=>s==+s?new Date(s*86400).toTimeString().split` `[0]:s.split`:`.reduce((a,b)=>+b+(+a)*60)*1/864e2
1
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JavaScript ES6, 102 bytes

s=>s==+s?new Date(s*86400).toTimeString().split` `[0]:s.split`:`.reduce((a,b)=>+b+((+a)*60))*(1/86400)