I am trying to solve some problems on converting from LMT (local mean time) to UTC (coordinated universal time) for flight planning purposes and I have some doubts. The problem is the following:
If the LMT in Delhi (LON=85ºE) is 17:45 (5:45 PM), what is the UTC?
First of all, what I've done is converting longitude into time as follows: $$t=85\cdot4=340\hspace{1mm} min=5h\hspace{1mm}40m$$
But then, in order to compute the UTC time, what do I have to do, either adding up this time to the current LMT or subtract it (and what is the reason)?
$$UTC=Delhi(LMT)\pm t$$
I have checked a website that allows you to convert from LMT to UTC (https://savvytime.com/converter/lmt-to-utc-india-delhi) and I've seen the solution is 11:15 PM.
However, if I add or subtract my result, it is not the same as the given in the website. I suspect it is because in India they are using UTC +5:30 and probably these equations do not contemplate that, but I want to know the opinion of the experts anyway.