A bitcoin service offering 5% a year can't be trusted unless they provide some very compelling info and there is nowhere legit offering anywhere near that. Do you even need to ask about 5% a day?
It's not paying out an interest rate of 5% a day. 5% compounding daily is 5% a year but the interest is calculated daily (whereas a typical mortgage is semi-annually).
Example: I have $1000 in an investment and it earns 5% a year, compounding annually. After 10 years I have $1628.89. If the interest compounds daily though, I would have $1648.66.
Use this formula for calculating future values:
FV = PV (1 + i) ^ n
FV = Future Value
PM = Present Value
i = Interest Rate
n = Number of compounding periods
When you're doing calculations for daily interest you take the rate of 5% (a year) divided by 365 (or 365.24 for the most accurate answer, as that's the actual length of the year) so 5 / 365 = 0.0136986301369
FV = 1000 (1 + 5) ^ 0.0136986301369
Follow BEDMAS (or grab a scientific calculator) and voila.
Now I do need to point out that 5% a year guaranteed is still very high (compared to bank/GIC rates at the moment) and I wouldn't trust it.
