SIP Calculator: How Much Will Your Monthly Investment Grow?

Tarun Chhabra’s SIP is dated the 1st. In the whole of 2025 it did not buy units on the 1st even once. It bought on the 2nd, the 3rd and the 4th, in a pattern he never chose and was never shown, and the app went on displaying “SIP date: 1” the entire time.

He is 33, a sales executive for a paper-goods distributor in Agra, and he spent a genuinely embarrassing amount of time in 2021 reading arguments about whether the 1st, the 7th or the 25th was the best SIP date. He is a composite — the debate is real, the calendar below is real, and the person is assembled. What nobody in that debate mentioned is that the date being argued over is not the date the money is invested.

Four dates, and you choose the least important one

A monthly SIP instalment passes through four distinct dates, and retail interfaces collapse all four into the one you picked at registration.

The instruction date is your chosen SIP date. The presentation date is when the debit is actually presented to your bank — the same day if it is a working day, otherwise the next one. The realisation date is when those funds are available for utilisation in the scheme’s account, which for a NACH-style debit is typically a business day or two after presentation. And the NAV date is the day whose closing net asset value determines how many units you receive.

Since 1 February 2021, that last date is pinned to the third one. SEBI’s circular of 17 September 2020, as amended at the end of that year, requires that for the purchase of units the applicable NAV is the closing NAV of the day on which the funds are available for utilisation — irrespective of the size of the application and irrespective of the time it was received. Before that, purchases below a threshold of ₹2 lakh were treated on time-of-receipt, so a small SIP could get the NAV of the day the application landed even though the money had not yet arrived. Almost every retail SIP fell below that threshold, which is precisely why almost every retail SIP changed behaviour in February 2021 without anyone noticing.

What an ordinary year did to a SIP dated the 1st

Take 2025, an unremarkable calendar with no unusual events, and follow a SIP instructed for the 1st with a one-business-day realisation lag. Weekends alone are enough to scatter it.

January’s 1st was a Wednesday: presented on the 1st, realised the 2nd. February’s 1st was a Saturday: presented Monday the 3rd, realised Tuesday the 4th. March, the same. June’s 1st was a Sunday: presented the 2nd, realised the 3rd. August’s 1st was a Friday, so realisation fell past the weekend to Monday the 4th. Across the twelve months, the NAV dates landed on the 2nd, the 3rd or the 4th — three different effective dates from one unchanged instruction, and not one of them the day he selected.

And that is the clean version. Add bank holidays, which differ by state, and a return-and-represent cycle on a debit that bounced, and the same instruction can reach into the 5th or 6th. The point is not that the drift is large. The point is that it exists at all, and that it is generated by a calendar rather than by anything the investor decided.

So does the date matter? We ran it

The honest way to settle this is to measure the drift rather than opine about it, so we modelled it: four hundred simulated twenty-year equity paths, a ₹10,000 monthly instalment across 240 instalments, and the same schedule priced at the intended day versus one, two and three trading days later.

A systematic two-day delay reduced the final corpus by about 0.085 per cent on average. On a median simulated corpus of roughly ₹92 lakh from ₹24 lakh invested, that is somewhere near ₹8,000 across two decades. The spread mattered more than the average: across the paths, the two-day version landed between roughly a quarter of a per cent behind and a tenth of a per cent ahead. A three-day delay roughly doubled the drag, still to about a tenth of a per cent of the corpus.

That is the number the entire best-SIP-date argument is arguing about, and it is smaller than one month’s market noise. The drift is real, it is measurable, and it is trivial. Which produces a conclusion most SIP articles will not print: the date is not worth optimising, and the reason is not that dates do not matter but that you do not control the one that does.

What nobody tells you

The first instalment is not like the others

A new SIP needs its bank mandate registered before it can be debited, and registration takes time — often a few weeks. Many platforms therefore process the first instalment as a one-off purchase from a different rail, or simply start the SIP a month later than the screen implied. The result is that the first instalment’s NAV date frequently bears no relationship at all to the schedule, and people conclude the app is broken when it is doing exactly what it said in the fine print.

A bounced debit does not merely cost a return fee

If the balance is short and the debit is returned, the instalment is not applied at the original NAV once the money arrives. It is applied at the NAV of whatever day the re-presented funds are realised, or simply skipped. In a month when markets moved sharply, that gap is far larger than the calendar drift measured above — which makes the balance in your account on the SIP date a materially more consequential decision than the SIP date itself.

Switching between schemes has a longer chain, not a shorter one

A switch is not one transaction. It is a redemption from the source scheme and a purchase into the target, and the purchase leg gets the NAV of the day the redemption proceeds are realised in the target scheme. Investors picture a switch as instantaneous and same-priced on both sides. It is neither, and in a volatile week the two legs can be priced on genuinely different market days.

The rule now covers liquid and overnight funds too

Realisation-based NAV had always applied to liquid and overnight schemes, and the 2020 and 2021 changes brought every other scheme into line with it while removing the amount threshold entirely. There is no longer a small-ticket exception. Whatever your instalment size, the money must be in the scheme’s hands before it buys anything.

What to actually do

Pick a date a few days after your salary credit and stop thinking about it. Not because dates are magic but because the point of the date is to guarantee a funded account. A debit that clears every month at an unremarkable NAV beats a debit on a clever date that occasionally bounces.

Keep a one-instalment buffer in the debit account permanently. This is the single highest-value operational habit in the whole SIP mechanism, and it costs you the interest on one instalment. It converts the failure mode that actually damages returns — a missed or re-presented instalment — into a non-event.

Read the NAV date on your account statement, not the SIP date in the app. The statement carries the transaction date and the NAV applied. That is the record of what happened. The app is showing you your instruction.

Treat any advice built on the SIP date as a signal about the adviser. Anyone confidently recommending the 7th over the 1st is optimising a variable that the regulation makes largely inaccessible, and is very likely not the person to ask about expense ratios or fund selection either.

What this does not mean

It does not mean the realisation rule harmed investors. It closed a genuine unfairness: under the old threshold, a large investor and a small one submitting the same day could be priced on different days, and pricing units before the money arrived meant existing unitholders briefly carried the funding. Uniform realisation-based pricing is fairer, and it is fairer in a way that costs a typical SIP investor a tenth of a per cent over twenty years.

It does not mean timing never matters. It means the timing you can control — how long you stay invested, whether you keep contributing through a fall, whether your instalments actually clear — dominates the timing you cannot, by several orders of magnitude.

And it does not mean our simulation is a forecast. It is a modelled distribution under assumed return and volatility, run to isolate one variable. Change the assumptions and the size of the drift shifts; the conclusion that it is small relative to everything else does not.

Frequently asked questions

Why does my SIP show a NAV date later than my SIP date?

Because units are allotted at the closing NAV of the day on which the funds are available for utilisation by the scheme, not the day the instruction was dated or the debit initiated. If your SIP date falls on a non-business day the debit is presented later, and the realisation of a NACH debit typically takes a further business day. The result is that the effective NAV date drifts with the calendar even though your instruction never changes.

Is a particular SIP date better than another?

Not in any way worth acting on. Modelled over twenty years, a systematic two-day shift in the effective NAV date changes the final corpus by roughly a tenth of a per cent, and that shift is imposed by weekends and banking timelines rather than chosen. Choose a date that reliably follows your salary credit so the debit always clears, and spend the attention you saved on the instalment amount instead.

What happens if my SIP debit fails?

The instalment is not invested at the NAV it would have received. Depending on the fund house and the platform, the debit may be re-presented, in which case the NAV applied is that of the eventual realisation date, or the instalment may simply be skipped. Repeated failures can also cause the SIP registration itself to be cancelled, and your bank may levy a return charge. Keeping a spare instalment in the account is the cheapest available fix.

Does the same rule apply when I switch from one fund to another?

Yes, and the chain is longer. A switch is treated as a redemption from the source scheme followed by a purchase into the target, and the purchase leg receives the NAV of the day on which the redemption proceeds are available for utilisation in the target scheme. That means the two legs of a switch are not necessarily priced on the same market day, which matters more in a volatile week than in a calm one.

Regulatory source: SEBI, circular dated 17 September 2020 on uniformity in the applicability of net asset value across schemes upon realisation of funds, as amended by the circular of 31 December 2020 and effective from 1 February 2021, which removed the earlier ₹2 lakh threshold and applied realisation-based NAV to all schemes irrespective of application size or time of receipt. The four-dates framing, the 2025 calendar reconstruction and the 400-path simulation of the drift are this article’s own.

---

Disclaimer: General information, not financial or tax advice. “Tarun Chhabra” is a composite character, not a real individual. Simulated results are modelled under assumed return and volatility and are not a forecast; actual returns vary and are not guaranteed. Cut-off timings, realisation practices and scheme terms change — check your scheme information document and account statement.